T R I S O M I C S I N 7)7SU~1 S A T I V U M D E . I V I e D FR~05~ A N I N T E R O H A N O ] ~ H E T E R O Z Y G O T E

I ~ T F,,ILEEN SUTTON

(With Twenty-one Text-f, gnres and Three Diagrams)

]. I~T~ODUOTZON

TEBS~ different ~ypes of trisomic have been found in D~tur¢ Stra~w~gu.vh and these were originally designated by Belling & Blakeslee (1926) as primary, secondary and tertiary trisomics.

]Primary trisomics have an extra chromosome homologous with one of the chromosome pairs in the diploid complement. Secondary trisomics have an extra chromosome consisting of two segments homologous with one another and with a segment of one of the chromosome pairs of the diploid complement. Tertiary trisomios owe their origin %0 segmental interohan.ge between non-homologous chromosomes, and the extra chromosome is made up of two segments which are homologous with segments in two different pairs of the diploid complement.

In P i s ~ sativu~ segmental interchange is frequent. The N line (structural type 1, Sansome, 1937) is taken as a standard line for chromosome structure, and several other lines have been obtained in which segments of two chromosomes are interchanged relative to those of the N line. One of the earliest recorded was I-Iammarhnd's K line (Hammarlund, 1929)] which is the subject of this paper.

~'~nen the K line and the N line are crossed together, a structural heterozygote (or more specifically, an interchange heterozygote) results, which has a meiotic association of four chromosomes, two being normal (N Hue) and two relativsly interchanged (K line). Sansome (193.3) has given the two normal chromosomes the formulae AXE, DF, the two interchanged chromosomes being AXD, ~,v; and I have adopted her formulae in this paper. The chromosomes AXE and DaY are about the same length, but the exchanged segments AX and F are unequal, so that AXD is considerably longer, and EF correspondingly shorter than sitter of the non-interchanged chromosomes. Diagram 1 shows ~he association of four; the other ten chromosomes form five normal bivalents.

The viable gametes from reguIar disjunction of this association wilt h~vs XXN and DF (N-line chromosomes) or AXD and EF (K-line

~60 T'riaomic8 i~ P i s u m s~tivum

chromosorces). Numerical non-dis iunction occurs occasionally, and gives deficient gametes ~vith six chromosomes anc~ (,~ + t) gametes with eight chromosomes. This has been described in studies of the cytology of the (K x N) interchange heterozygote by It&kansson (1929, I931).

The (,~+1) gametes from numerical non-disjunction can combine with either ~ype of ordinary gamete formed by regnIar disjunction, to give a trisomie plant. Consequently, a given (n+I ) gamete can con- tribute to two different kinds of trisomie, ~'or instance, if numerical non-all@motion takes place ~s indicated in Diagram 1, a gamete will be

E F

f

D

Diagram 1. Cbx'omosomes of ~he X ring. The dotted line indie.~tes on~ of the ws ys in which nunierie~l non-disjunction rn~y occur, giving two g~metes swith the E27' segments in excess ~nd two g~me~es deficient in these segments.

formed with chroxnosomes AXE, f)F, EF (namely, the N-line haploid complement+ E l ) . With an fg-line (AXE, DP) gamete, this wili form a ~ertiary ~risomic of the constitution N-line structural homozygote + E ~ ; bat with a K-line (AXD, ~F) gamete the resultant trisomio will be: ( K x N ) interchange heterozygote+EF. This type is ~og strictly a tertiary, as the extra chromosome is completely homologmls with one other chromosome of the comElemen~ , nor is it sgrietty a primary, as the extra chromosome has segments homologoas with two other chro- mosomes of the complemeng; in gkis .paper the type is called a ~risomic interchange heterozygote.

~our t?loes of tertiary, with eo~responding tHsomio interchange heterozygotes, are theoretically possible, as numerical non-disjunction may occur in four differen~ ways, according ~o whiok of the four chro- mosomes fails to disjoin from its neighbours.

E~LEEN STJTTON 4~61

9. ]V~AT:~I%IAL AN]9 i~ET~IOI)S

In 1934 ~'iiss Pe)lew made a ero~s between an (MI x N) interchange heSerozygoSe and a K-line interchange homozygo%. The female parent (MI xN) had one set of chromosomes corresponding to those of the standard N line, so that some of the f l ~vere (K x N) interchange heterozygotes. These plants were easiIy distinguished from (Mtx X) he~erozygotes, for in ~hese, two associations of fern' chromosomes are formed (Sansome, nnpublished) and they show appro~mateIy 75 °/o of

TABLE I

3II interchange heterozygote >: K interelmnge homozygot:e ster'ile fert, ile

i443,/34) 151i~/34)

g~m erie types MI N ' - . < / K

"PI (M[x X) double (K x N) interchange interchange hetero- heterozygotes

zygotes ~ sterile ½ sterile (~ g99s)

i I ga.metie typ#s K. .N

F 2 B-M/36 and 547-55t/36 K interchange born'o- \ / ,\T structural homo-

zygotes, fertile ~ z~g0tes feitJe

(K × iV) intereha.nge he~erozygotes, sterile

t . . . . . . . . J

including tertia@ trisomiea and trisomie intereha.~ge heterozygotea

Fa's 1-2~/37

gametic sterility. It was in the P~ obtained by selfing one of the (K x N) heterozygotes theft t¢isomies were first s~.udied. The work was extended to iV3's obtained by selfing four of these trisomies, k pedigree is given in TahIe I.

R.oot tips were fixed in La Cent's 2BE and whole flower buds in Carney' s ~xative and La Cent's 2B. P~oot-tip sections were cut at 16-t8/~ and bud sections ag 22-24~. All seegions were stained by the gentian ~dolet-iodine method.

Journ. of (deneties xxxvIa 30

462 Triao~nics in P i s u m s a t i v u m

The drawings were made with a eamer~ lucida at an initial magnifica- tion of 3500 diameters. Figs. 1-7 have been reduced in the text.

~. OBSE][%VATION8

£'o'magc divi,sio~s 'i~ te~ f a m i ~

Fifteen of the eighty-five £a plants (i.e. abottt 18%) were retold by somatic chromosome counts to be tsisomics, with fifteen chromosomes instead of the usuM diploid number of fourteen.

Netaphase plates were studied in ~he root-tip preparations of diploids scored by ~iiss Pellew as fertile. I t is fMrly easy to pick out a pair of small chromosomes with a subterminal constriction, and this pair served as a mark with reference to which two types of chromosome complement could be distinguished. One type has another pair of chromosomes smaller still, without a very clearly marked constriction; the other type lacks this smaiIer pair. The difference can be seen in Figs. 1 and 2. The very small chromosomes are the E F ch'omosomes of the K line, and are accompanied by a pair of long i X D chromosomes.

In diploids scored as hail-sterile, both the small chromosomes wRh the marked constriction can be see~ and there is always one E F and one A X D chromosome, without similar partners (Fig. 3).

In the trisomics the pair of small constricted chromosomes was always visible, except in two plants in which fixation was poor. tn plants with well under 50~/o of gametic sterility (on pollen counts) one E F chromosome was observed; in plants with about 50 °/o sterility, l,wo EF chromosomes could be seen (Fig. 5). An exception was found in one plant (1~6/.36) which had two EF chromosomes, though scored as only 25 °/D sterile on its pollen. On the ovules, however, it was scored as half- sterile, and its breeding behavioar was tha t of a hMf-sterile (see below).

Somatic divisio'~.~ in ]?a ]~ ig ies

The results of selfing four I~'~ trisomies are given in TaMe II. The progeny of Es/36, in which, half-sterile plants occurred, confirmed

its ]lalf-sterility as judged by pods and chromosomes. Of tee four trisomics it gave, one (IS/37) was a new type, di~ering from those described above in having three E F chromosomes (Fig. 7), Its pollen was found by 3:liss Pellew to be about 28 °/o bad.

The trisomic (5s/37) from ¢4t/36 resembled E~/36 in having two EP chromosomes, though it was scored ca pollen as only 23% sterile. No record was obtained on the pods, as most of the flower buds were fixed,

EILEEN SUTTON 463

but a ring of four chromosomes observed at meiosis showed that the plant was an interchange heterozygote.

From G~/36 the trisomic t7a/37 had one root tip with only fourteen chromosomes, the normal N-line complement. Seven other roots showed fifteen chromosomes, and two more had fi£t.een chromosomes in the majority of cells, with a section in which the cells had thir ty chromosomes. In these two roots somatic doubling had occurred, and .in the first the extra chromosome had apparently been eliminated.

TABLE ii

P~-ogeny fro~ sely%cy o~'.ig'i~~a~ t~isom, i~, sho~vin 9 degree c,f ste~'.i~ity a.nd 9~u~~be.r of s~nall (EP) &~.omoso~l~es found i,~, tt'isomics

[Family Na~m'e of progeny .llOS~ o f c ¢'

]Parent progeny I)iploids Trisomios ATe,

E"/36 1-2/37 3 fertile 1 d fertile (? INN) In/37 d 25 % sterile 6 ½-sterile 1 ~ fertile (3NJT) 18/37 9 ~ sterile 2 d ½ sterile (2E/7) 1737 (2L'F) 2~I37

*Glf36 3-7/37 4 fertile I ~ 23% s~erile (2E}') 5~737 <2 ½ sterile 4[ ½-s~erile (2E~)

*0"-/36 8-IS/37 20 fertile 6 ~ fertile (I/~f) 111/37 c~ feifile 12V37 (IE/~) 14~/87

16 ~] 37 17V37 I7~/37

? E~/38 19-22/37 7 fertile 1 ~{ 4I % sterile (2,Eft) 21~/37 <? ~- s~erile (2EF)

Total @4 plan~s 12 plants

* The seeds fro~ which these plants came had peet~ar starch--see _.'%fiss Pellew's p~per.

Families 19-22/37 were of doubtful parentage, as some of the labels were unfortunately mixed in planting out the 1936 family. The parent was probably E~/36, which was a half-sterile trisomic, and although the seven diploids in this group were all fertile, the single trisomic 211/37 had two E/7' chromosomes and a high sterility.

There were twelve trisomics in a total of fifty-six plants (about 21%) in these families. The " fer t i le" (tertiary) trisomic gave six trisomies in a to~,al of twenty-six plants, while the three trisomic interchange ]aetero- zygotes together gave six trisomies in a to~al of thi r ty plants. The proportion of trisomics found is discussed in § 5.

30-2

464 T'riaomies in Pisum sa~givum

X j , Figs. l -7. Somatic me~apha.ses from roo~ 9i]?s~ sm~ll cons'br[ct;ed c~'omosomes marked a,

~,he sm~Iler chromosomes m~rked b. 1. @a/36, ~q, struo%~m~,]ly ]~omozygous diploM ([( Iino}. ~. DS/36, a s~rudsta'alJy homozygotm cJJploid (!Y line}- 3. F7/3t, ,~ s~m~urally het;<~rozygous diploid (N x I(}. 4. E'~/36, ~ sbruct, ur~d homozygo~e wi~h cxtra chromo- some. 5. G1/36, ~ s~ruc~ura.1 heterozygote wi~h extra cl~'omosomo. 6. 3231=/36, '~risomic, of I~pring of an JlfI stru~bur~l he~erozygo~e. 7. ts/37, a pr imary trisomic derived from ~ strttcgtu'al heberozygot~ wi'~h exbr~ chromosome.

. E I L E E N S U T T O N 4 6 5

Meiotic divisions Pollen mother cell divisions were studied in material ~¥om some of

the fifteen Y~ trisomics, and Table I I I shows the different conllgm:a~io~s found, some of which are drawer in Figs. 8-21. Some o:f the possible configurations in a trisomic interchange heterozygote are shown in Diagram. 2; analogous figures for a ter t iary trisomic were given by Belling (1927, Diagram 5}.

TABLE III

Meiotic ch,ro.moso'm,e at,range,merits ~i~7, terti~'y t~'iso,mics and trisomic inte,rcha~ge heterozygotes

Tertiary trisomiea Chroreosome

Pl~ngs ex~mh~ed arrangements Text-~gures 7~ a, X s and 548~/36 7 II + l I :Figs. 8, 9

6 I I+] . III (chain) :Figs. 10, 11 (one EP eln.omosome) same <rfl~g and rod) Fig. 18

5 II+l V Figs. 12, 13, 14 5 I I + t I I I + 2 I 5 I I + l IV+I I :Fig. 15

Plants ex~m[ned G 1 and E~/86

(~wo ~.F chromosomes

Trisomic interchange heterozygoges Chromosome arrangements

(?) 7 II41 1 6 II + I llI

5 II+l IR~+l I (ring of fore') (chain of four)

5 I I + I V

Text figm'es and diagrams

Figs. 17, 18, Diagram 1 (6-t0)

Fig. 19, Diagram 1 (5) :Pig. 20, Diagram 1 (4) Fig. 21, Diagram t (1-3)

In the tertiary trisomics (E 4, IK 6 and 548~/36) about 35% of the cells had seven bivalents and one univalent (80 cells out of 22.9) and ¢9% had six bivalents and a chain of three (112 cells out of 229), The other configurations were not common, and the last was seen only once.

In She t~isomic interchange heterozygotes (G I and I[s/36) one cell only (in G1/36) was seen which appeared to have seven bivalents and one univalent.

First anaphase, examined in X.s/36 and @1/36, shewed an occasional univalent lagging on the spindle equator, and sometimes this chromosome was dear ly split. There were few divisions at this stage, but connts were, made from the two p]ants which, showed that the univalent is excluded in about a ciuarter of the first divisions :

Cells with Plan~ no. lagghlg univalen~ Normal cells Total Xff36 18 (27 %) 50 (73 %) S8 GVs6 9 (24%) 2~ {v6%} as

466 T~'iso.mics 'i'~t P i s u m s~¢eivum

1

F~ / @A

F E )E

5 E _@F

" 0 g D

3

F ~ E

6

Diagram 2.

7 8 ....... . 2 ~ -

Somo of [he possible meiotic co~figur;~ti.ons bl au i~ferchcnge he~erozygo~e wJ~h an ex~;r~ ohromosom.e.

8 9 I0

n {

]5 16 17 s

" 19 : 20 2I

Figs, 8-]6. N[eta~hase of first meiotic division h~ strueturM homezygo£es w~,h an extra chromosome; abnormM cenfigm'ations in black, normal biva.le~Its hl outline. S and 9. :E~J/3@,'aeven bivalents and one univMent. I0 and ]]. E~/3(i, six biv~lents ~nd chain of ~hree. 12 and 13. E4/~6, ~ive bivaIents and chain of five. 15. E'J/36, live bivaleats, chah~ of ibm' and one univMent. ]5. 5~8"/36, "ring and rod" association of three.

Figs. ]7-2L 3I¢i~aphase of first meiotic division hi G~/36, ~ structural heterozygote with an extra ehro~.osome. 17 and 18. Six biva.lents ~nd ~ ehMu of ~hree. 19, :Five bivalents, ring of font and oue univalen%, ~0. Five bryn]earls, chain of four ~nd one u~iva]ent. 21. Five bJvMents and chain of five.

'4:68 Trisomics in P i s u m s a t i w t m

In one trisomie itlterchange heterozygote, 5507/36, examined later, a few cells at first anaphase showed a long dicen~ric bridge and a small fragmen% presumably due to inversion crossing-over. A chiasma is formed i~ the inverted region in only 4% of the cells (total count, 32 bridges in 789 cells). The inversion is therefore a small one near the distal en.d of the longer arm of the chromosome. Divisions at first anaphase were studied in several other plants of the sam~ family without finding any sign of inversion. H~kansson (1931) has also recorded ohromadd bridges in Pis~m sativum.

{. I~U~IERICAL N-ON~DISJUNCT][O~N- IN THE I{ RIN(4

I t has been st~ted that two types of chromosome complement can be distinguished in mitotic metaphase plates of the fertile diploids-- (a) with and (b) without a pair of small E F chromosomes, and tha t these types are ~haraoteristic of the homozygous I{ line and i¥ line respectively.

There are also two tyt~es in the F., trisomics, the first with one and the second wRh two E& chromosomes. The first are relatively fertile and have no closed ring of four at meiosis, and they are presumably the (b) type of diploid (N line) wRk the small EE chromosome extra (tertiary trisomics). The second type are usually about half-sterile, and are ( K x N ) interchange heterozygotes havihg the four chromosomes-- AXE, E_E, _FI), DX-4, Which cau form a ring at meiosis, and a second small BE chromosome in addRion (~risomic interchange heterozygotes).

The fact that the EF chromosome is always the extra one has two possibie explanations. Non-disjm~ctlon in ~he K ring may take place in o~e way only, so that chromosomes AXE~ EZV, FD go together to one pole; that is, the small chromosome fails to disjoiu from either of its neighbours, w]].i].e the longest of the four, A.XD, disjoins from both its neighbours, as in Diagram t. Alternatively, if all fern: types of non- disjunctiml occur, an extra AXE, AXD or DF chromosome mus~ cause inviability at the gametic or zygotic sts,ge.

The seco~.d possibility may be dismissed, as it w e n d require at least ~5 % of numerical ~on-dis] unction, which is far in excess of H~kansson's observati.ons (1931).

Numerical l~.Oz~-d.lsjunction therefore occurs in only one of the four possible ways. As a resuR of this, all the tertiary trisomics are expected bo be N-line plus o~le EF chromosome, all.d tertiary trisomics with the komozygous K-line complement are not expected. The F~ rest.fits agree with this expectation.

EILEEST S~J'r~o~ 469

5. ~A3fETE8 AND O~SP~gING ]rROS,1 TKE ff~ TgISONIOS

The proportion and kind of Q~+I) and of viaMe gametes to be expected from the i~ ~risomies will Ice af%eted by

(t} the distribution o:f univMents, (:~) non-disjunction of multiple associations, (3) the eliminatio~ of univMents at meiotic anaphases. These three heads will be considered i~l turn. The chromosome constitutions of the two t]q~es a r e :

Te:.l,iar~r tMsomics A.X,E E ~ , / : D + 5 m:spech'ied paix's " AXI~ :" . ' ~ D

Trisomic ixltorchange he%~rozygo~es A X E . E1C YD. 2XYA + 5 u:aspecified pairs

(l) Tl:m univatents seen at meiosis in both. kinds of trisomio are not necessarily the BY et~romosome. In tertiaries, the E segment may pair with that of an AXE .chromosome or the _F segment may pair with that of a DF chromosome, so ~hat the free univalent may be either AXE or DF. The univalent will be distributed a~ random either together with i~s homologue, or with EF. The fit'st alternative will give gametes with AXE or 2)F dup]icated, and by combination with normal gametes with chromosomes AXB. DF, .primary trisomies will be formed in which the extra chromosome is homologous with a single pMr, instead of with two half-pairs, of the normM complelnen~.

In the ~risomic interchange heterozygotes the effect of univalent distribution is complicated by the fact that the univalent rarely if ever occurs in a cell with seven paired chromosomes, but usually with a chMn of three or a ring or chain of four. The new kinds of trisomie which might arise could a]so result from non-disjunction of an association of three, four or five chromosomes, and witI be considered in the following section on nma-disiunc~ion.

(2) In the tertiaries, if three chromosomes forming a chain disjoin regularly front one another, as in the trisomics of Da.g~r~'a described by Belling & Blakeslee (1924) and in Pd~oades' extra-fragment maize (19-36), the pair of completely homologous chromosomes (AXE or/)F) separate, and the extra chromosome EF also separates from that to which it is attached, thus :

Y E A X ~ or FD awN \ / x, /

BXA DY

The (~ + 1) gametes therefore have the same extra chromosome as the parent.

If non-disjunction occurs, the resulting gamet.es combining with

haploid gametes (AXB.DF) will produce pnimary trisomies (extra chromosomes A X E or DF) like those described ~bove. When ~on- disjunction occurs in a chain of four or live (FD.DF. FE.EX_4. AXR) the same kind of gametes and primary trisomies may be formed, if a homologous pair from one end of the chain goes to the same pole as one of the pair at the other end; non-disjunction of the middle three chro- mosomes witl give the usual (ETa) tertiary type of (.~z + 1) gamete, while non-disjunction of the end three dlromosomes will give gametes (probably inviable) lacking one or more parts of the chromosome complement. The number of primary trisomics and inviable gametes to be expected from the chain of four or We is very small, since such associations are not OOl~nm o n .

In trisomic interchange heterozygotes the chain of three is probabIy most often that represented in Diagram 9. (6), the extra chromosome B/~ pairing wi~h its homologue to form a normal bivalen< wtdle the ottler three chromosomes P D . D X A . A X E form the chain. Non-disjunction will give rise to two types of (~+ I) gamete, one with the chromosomes Exv.~/XD.DF and the other with E_g..4XD.PXE, together with gametes (probably inviable bseaus~ incomplete) with Ef..~ftXB and .EaV._FD. The (~+1) gametes may take part in forming new tertiary trisomios with the DrY or A X E chromosomes extra. The fourth tertiary trisomic (with the AX.D chromosome) may come from non-disjunction of a chain of five (Eie .FE.EX,4 .AX.D.Df) if this occurs so that the three end chromosomes EX.4.AXD.DzV stay together. The primary trisomic with E F may also come from this chain if a gamete is formed wi~k E F . E F . A X D (this may abe come from.regular disjunction of a ring or chain of four, if the univalent passes to the same pole as the BT~.z[XD chromosomes from the ring or chain).

Nonodi@metion of the rj.~g of five will produce some incomplete gametes, and the ring of four is expected to give about 50 ~ incomp]ete gameees by non-disjunction; however, 5he univalent EF might perhaps compel~sate for this incompleteness if it were included in a gamete with either the A.XD .AXE or ~tXD.D/e c.hromoso£FP.s. The total vial~ility of gameees from trisomic interchange heterozygotes would not be expected. ~o be less ~han 50 %.

The types of trJsomies that ma, y appear i~ the progeny of the original tertiary t dsomies and trisomic interchange heterozygoges are s~hown in. Diagram 3, and. are as follows:

From the tertiaries, primaries with ,!IXE (:3) and D/~ (2) and the original tertiary with EF (1).

1 !

°1 ! %

© o

I t ~.~j L~

-~ ~ ,,~ -~ ~ . , ~

r3

2

,.. %

o

r

t

EIL]~EN SUTTON 471

From the trisomic interchange heterozygotes, the primary with E F (9), tertiaries with A X E (.5}, A X D (10) and DF (7) and the original t~'isomic m~erchgnge heterozygote with )~F (4) ; also interchange hetero- zyggtes with A X B (6), A X D (11) and DF (8).

As recorded above, the trisomies in F8 families were mosdy the same as ~he two parental types (Diagram 3 (t) and (4)), but ts/37 proved to b6 a new type of trisomie (Diagram 3 (9))--the only primary trisomio we ]iavefonnd in P~s~m, except that recently reported by Keller (1938).

(3) A univalent was found to be excluded at anaphase in about a quarter of the first meiotic divisions, which means that the proportion of 's to 0~+1) gametes will be inca'eased to 5 : 3 (37.5~ (~ +1)) instead of being t : 1 (50% (~+1)).

~n the tertiary trisomics, faihre at the gametophy~io stage (pollen grai~ls) w~s found by Niss Pellew to vary from about 5-30 %. The upper timR is a good deal higher than was recorded by ]31akeslee & Cart.ledge (I926) for the tertiary trisomic "Wiry" in/)at~ra, and this may be due (a) to incomplete chromosome complements arising from non-d/sj unction (which is known to be more frequent in Pis~.~,m than in D~t.~'a, perhaps because of nnterminalized ehiasmata), and (b) to reduced viability of the (~+I ) pollen grains. The latter cause cordd account for the whole of {he bad pollen, since it has been estimated above that about 37.5% of the total number of pollen grains are (~z +1) grains. There is no evidence so far that the extra chromosome is ever transmitted through the pollen, so that even those (~ + 1) grains which appear sound do not fertilize the ovules in competition with ordinary haploid grains.

The amount of b~d pollen in the trisomic interchange heterozygotes varied from about 500/0 (as expected) toas little as 25 and 230/0. There is no ob~dous explanation of the low sterilityjn two plants.

On the female side, a higher proportion of the (~ + 1) gametoph~tes are ftmetional. With no loss, 37-5 % of (~ + 1) ovtfles would give 37.5 % of (2~+ 1) progeny. Twenty-one trisomies would be expected in a total of fift~-six plants, whereas the actual number obtained (from both types of triso]lxic) was twelve in fifty-six. This indicates that about 57 % of the (~+ 1) ovules develop after fertilization.

O. GENETIC [RATIOS IN TI~ISONIOS

Single-gene ratios in dip]oids from the K-line interchange hereto- zygotes will be normal (] : 1 for a back-cross, 3 : 1 for a self), but. among the trisomic progeny the ratios for genes situated in the interchanged chromosomes may be quite different.

Trisomics in P i s u m s a t i v u m

These ratios will wlry, first according to whether the gene is present in three doses (i.e. lies im segment E or F) or in two (i.e. lies in segment AX or D), and secondly according to the number of chiasmata or cross- overs between the gene and the centromere.

It may be pointed out that in interchanges where the distanc'e between the centromere and the breakage point is conMderable, and a. median chiasma is formed in this region (Sutton, 1935) numerical non-disjunction is not expected; it may occur in a simple ring or chain of four, but not in a £gure-of-eight of four chromosomes. Hence for the present purpose, when the breakage point lies between the given gene and the centromere, the number of chiasmata between gene and centromere is the same as f, he number of chia.smata be~we.en gene and breakage point.

}I&kansson's study of the K ring (1931) showed that ~gures-of-eight were not formed. At metaphase there are usually two chiasmata in each of the segments i X and D. One of the segments E and F has usually one and q~casionally two chiasmata, while the other of these segments has either one or none at all (in which case a chain is formed instead of a ring). As there is little termJnalization of chiasmata in P~su~, we may take these numbers to represent the number of ¢hiasmata actuM]y formed at prophase.

Table IV summarizes the range of single-gene ratios expected. Colnmn 2 gives the ratios expected among the (9~ + i) gametes, according to the number of chiasmata between gene and centromere. The general principles on which they are worked out are illustrated below.

Suppose that a gene Y and its allelomorph y lie in the E or F segment, so close to the centrom.ere that no c hiasma is fomned between gene and centromere. The second meiotic division, following a non*disjtmctional first dJ.vision, will then be equational for two d~roma~id pairs, YY and yy, in the same cell, and all ('~+1) gametes will consequently have the constitution Yy (Table tV A, [.rst ]ine). If, however, one chiasma were always formed between gene and een.tromere, the second division would be reductionM for two Yy ehromatid pairs, giving (n + i) gamete~ in the ratiolYY : 2Yy : lyy (or 3Y : ly, Table IV & second line).

~/gith two or more chiasm.ata between gene and centromere the pro- portion of equational to reductional second divisions ~/11 gradually approximate to ½ : ~. For a given num.ber (~) of ohiasmata, the proportion of equationaI second divisions (E2 '~) and redttcionM second divisions (R#9 are given by the formulae

E # = 1 - ~ [ 1 - ( - ~),,], R=,, = .~ [1 - ( - ,})q,

EILEEN SUTTON 473

These equations have been derived by tPIatl~er (1935), who gives the proportions (g" aud R ~) of equational and reductional first divisions for a given number of chiasmata. As the second division is of opposite nature to the first, the above formulae are the reverse of Number's formula (i.e. Ez'~=R ,~ and R,, '' = E ' 0.

Now an equational second division of an (,~+1) nucleus with two chromadd pairs YY and yy results in gametes which all flare the con-

TABLE IV

]3~:peeted si,r~#le-[actor ra~ios j'~'o~ an {~terd~s,rWe heteroz~gote ~o]~ioh is cds'o }~eterozy.9ou, s jbr a 9e~e Y / y ¢+~ one o/" #ae #~terehcm.cy~d el~'omo~omes, Tlw ratio Y : y ¢~, (~ + 1) ga~netes (e& 2) &pe~cls on the ~.~mber ¢ chias~,c~tcb ~)ro.~imcd to tim ge'~~,e (eo~. ]). CoL 3 gives tlse ratios e l Y : y amo~ W the t.r~so~nie proge~.q whe'~ the heterozjgote is used as fema~e ?a, rent in a bc~e],:-cross, and col 4 gives the corres3m~di~g .ratios ~ohe,n tlze heterozygote is semi'cal,

No. of Back-cross (hetero. chiasm~t~ zygote as o) S d f

betweou gene {n + 1) gametes ratio Y : y ra~io Y : y and centromere ra~io Y : y ~n ~risomics in ~cisomies

k . Gene Y/y in extra, chromosome segment .g' or F 0 All Y All Y All Y 1 3Y: ]y 3Y:ly 7Y:ly 2 " 7Y:ly 7Y:IF 15Y:ly ~ i-t- [i -(-j ~'~ ].. l- ~-[~ -(- ~ ,.),~] -. ~ o-t-it_(. ..... ~, j:

[I - ( - })~] {- [~ - ( - l-)~] ~ [~ _ ( _ ½),] ]3, Gene Y/y in segment --iX or 2) (classes as before)

0 {a) a.ll Y or (a) all Y or (a) all Y or (b) a l l y (b) a l l y (b) l¥:ly

1 or more IY : 1F iY : ly 3Y : ly

stRu~ion Yy, while a reductional second division results in a gametic ratio of 3Y : ly. For a given number of chiasmatal therefore, all of the equational dRdsions and three-quarters of the reductional divisions will give Y gametes: and the remaining quarter of reductional divisions will give y gametes. The proportions,(Y" and y'~) of Y and y gametes for a given number (~J of chiasmata can thus be obtained from the formulae

In the limit, when ~a is infinite, the proportions Y'* and y'~ will be ~ and ~. As ~he number of ch~asmata increases, ~he ratio of Y : y thus approxi- mates to 5 : 1.

I f the gene and its allelomorph lie in the A X or D segment, the Y : y ratios in' ('n + 1) gametes will depend on the disjunction of a single pair

¢74= 'r,risornics i~ P i s u m sat ivtma

of shromatids. When no proximal ehiasma is formed, the second division will be equational. 0nly one allelomorph of the gone will be present in the (~,+ 1) daugh%r cell from the first division, the other having been diminated with the AXD chromosome in the (~-1) cell; therefore the (n+l) gametes may be (a) all Y or (b) all y (TabIe IV s, ,first line) according to which allelomorph is thus eliminated.

One ckiasma between gene and eentromere will result in a reductional second division for a Yy chromatid pair, which will give equal numbers of Y and y in the (~ + 1) gametes (Table IV B, second line).

With two or more skis.smuts, ¥ and y gametes will still Joe formed in equa[ numbers ; for all reductional second divisions will have this result, and equational divisions will involve a yy chromatid pair as often as

g YY pair. In a back-cross of a Yy int.erehange heterozygote bee (~ + 1) gametes

will only give rise to trisomics if the heterozygote is used as the female parent; and the ratio of Y trisomies to y trisomics will be the same as that for the 0~+l)'gametes (Table IV, col g). In a self, the (~,+1) ov-utes will have equal chances of being fertilized by Y and y haploid pollen grains; the resulting ratios are given in Table IV, col 4.

These ratios provide a new means of judging the ~pproximate position of genes i~ relation go the centromere, and locating bhem in one segment or the other of the interchanged chromosome.

Abnormal single-factor ratios will atso be obtained in the progeny of trisomies which are heterozygous for genes located in the extra (EFJ chromosome. These ratios are given by Sansome & Philp (t932, Table 3t) for trisomics wNch are simplex (Xxx) or dnplex (XXx) for a given gone. The effect of single-strand crossing-over (shroma~id segregation) is also discussed, and is further considered by Rhoades (19.33). It is therefore unnecessary t,o deal with these ratios here.

7. ~O~IPARI~0N OF TI~ISO~II08 IN ~_rSU!]£ ,TATZVKY!3Z

Apart from the single primary (P/37) the trisomies described here are of the types (2) and (3) described by Sansom.s (1933), namely, :~(2) a struetu:ral homozygote with an extra chromosome pairing tip with bwo bivalents, (3) a single-interchange hetsrozygol:e, forming an assoda- bion of four chromosomes with an extra chromosome ¢omposec~ of two of tee segments in the ring", One of the trisomics described in tier paper (&5,fn/.32) may well be of type (3), a trisomic interchange heterozygote, and may indeed, have the same chromosonm complement as ours, as it arose from a ring of six in whisk the K Iine was involved..

EILEEN SUTTON 475

H.~kansson (1936) has made cytological observations on a trisomic which has distinctive morphological features a.~d occurs rather less frequently than those described here, in a ratio of about one trisomic to nine normMs. The extra chrom.osome probably comes from the N tV ring (}!~.kansson, 1934:), though no association of five chromosomes was found at, meiosis. In one cell only, a ring of four -was seen; if the extra chromosome were independent of the ring, the N IV interchange heterozygote should show an association of four in nearly every cell, while a structural homozygote should not show a ring at all, even if the extra chromosome came from a ring in the parent, i t therefore seems likely that H&kansson's trisomic was of type (3), and tha~ intervention of the extra chromosome nearly always resuRed in the formation of a bivalent and a chain of three (or two bivalents and a univalent) in place of the ring or chain of four claromosomes.

Lu~kov (I937) has obtained trfsomics from a half-sterile plant wRh an association of four chromosomes, in which the interchange was induced by X-raying the pollen of the mate parent. He states tha t ';in the chromosome sets of these plants (the trisom_ics) there are no chro- mosomes which have undergone interchange, since we found no chro- mosome rings at meiosis". This does not necessarily follow, for even if the extr~ chromosome was derived from the interchange ring, the trisonlics might all be of the tertiary type, or the trisomic interchange heterozygotes might resemble ~ilcansson's in rarely (if ever) forming a ring of four at meiosis. On the other hand, two of his drawings show a configuration resembling a closed ring of three chromosomes. This could only occur in a "secondary" trisomic (Belling & Blakeslee, 1926) with an extra chromosome consisting of a single reduplicated segment, and the formation of such a trisomic would necessarily involve a structural change distinct from the interchange manifested in the parent plant. I f his figures do represent closed rings of three, Lutkov has a new -ldnd of trisomic (a secondary) in Pisum, and his statement does not ta.ke this into consideration.

In P'fsu~ anti'rum tertiary trisomics and trisomic interchange before- zygotes have been derived from Hammarlund's K-line interchange heterozygote, which has a meiotic association of four chromosomes. The origin of the trisonics [s shown to be in numerical nomdisjunction of the association of four in one only of the four possible ways.

~teven different types of trisomic may occur when the two original

476 Tdson~fcs i~ Pisum sativmu

types are selfed. Progenies of selfed ~risomJes are described, including one new (primary) trisomio.

The degree of sterility of the trisomios is discussed. An analysis is made Of single-gene ratios expected a m o n g trisomies

in the progeny of interchange heterozygotes These ratios p rov ide a means of locating genes in the in terchanged chromosomes.

Other trisomies reported in P . sa t iw~n are reviewed.

Pd~FEI~ENCES

B:ELLI-~G: J. (1927). ':The attachments of chromosomes at the reduction d/vision in flowering plants.". J. Goner 18, 177.

BELL]:~G, J. & }~LAKESLEE, A. F. (1924). "The configmrations and sizes of the chromosomes hi the ~riva.lents of 2S-chromosome D~tnr~s." Bros. nab icad. 80i., Wad~., iO, 116.

- - (1926). "'On fihe afita.ckment of non-homologous chromosomes at the reduction division in certain 25-chromosome Da.tu~-as." P~oc. hal _4sag. Sci,, Wa~A., 12, 7.

BLa~dK~,SL~.]~, A. ~.. & CA-~TLEDGE, J. L. (1926). "Pollen abortion in %he chromosomal types of Da~ura." iP~'oc. ~mt. icad. Sci., TEazle., I2, 315.

H.i~-~_~sso._v, A. (1929). "Chromosomenzinge in Pi.~u~ und ihre mutmhssliohe genetische Bedeufiung." tfzredi~aa, Lung, 12, i.

- - (1931). "~ber Chromosomenverkehfiung in Pisum." Herez{if~a, Lund, 15, t7. (1934). "Chromosomenverbind~mgen in einigen Iqreuzungen zwischen h~!b-

sterfien Erbsen2' IIereg{tas, Lund, 12, 210. - - ( i 9 3 6 ) . "Die Zytologie eines trisomischen P/sum-Typus." He'reditas, Lund,

2I, 223. H~a~¢~U~,TaT~D, C. (i929). "DrRte Mitteihmg ~ber einen Fail yon Koppelung und

freier lilombination bei ]Drbsen." Hereditas, Lung, I[2, 210. I(OLLEB, P. C. (].938). "Asynapsis in Pisum .scot'rum." J. GeneS. 36, 275. LuTKOV, A. N. (1937). "l~eciprocal transloo~tions ~nd gene mutations in B fsi~m

saC{rum induced by X-radie~tion of pollen." Bul l a22L igor. Set. rr (7), 977. M,~TI¢~, Ii. (1935). "P, eduotional and equational separ~otion of the chromosomes in

biv~lents and mtdtlv,ulents." J. Goner. 30, 53. ~O,~D~.S, ]~'L 5{. (t936), "An experhnent~l ~nd %h~oretical study of a chromosome

fragmeat in maize." Genetics, 21, 49I. S ~ s o , ~ , E. R. (193~). "Segmental interchange in Pisun~ sativum." Cyto[,ogia,

Tokyo, 3, 200. --- (1933). "Segmental interchange in Pi,~m. II." Cy~e~ogla, To]c~jo, 5, ].5. - - ( 1 9 3 7 ) . "Segmen~a| interchange lines in Pisum sc&iv.~o~. '~ zhra~cr~, Lend.,

i 3 9 , 113. SA~SO~E, F. W. & P:~Le, j. (1932). l~ecen~ Advances i'n Plant Genetics. London:

Churchill. Su'~:o~, E. (1935). "I-l:alf-d.lsjunction. in an associahon of four chromosomes in Pi~u~,~

sal, ivum." Ann. tYot., Long., 49, 689.