t/t-locus in the mouse - genetics

GENETIC CHANGE IN MUTATIONS AT THE T/t-LOCUS IN THE MOUSE

DOROTHEA BENNETTI, L. C. DUNW3, AND KAREN ARTZ’P

1Department of Anatomy, Cornell University Medical College, New York, New York 10021

2Neuis Biological Station, Columbia Uniuersity, Iruington-on-Hudson, New York 10533

Manuscript received December 8, 1975

ABSTRACT

Recessive lethal or semilethal alleles at the T/ t locus in the mouse generate new I-variants, with characteristics different from the parent allele at a rate of about 10-3. Almost invariably the variant chromosome carries marker genes derived from the opposite parental chromosome. New t-mutations obtained in this way are sometimes recessive lethals that are indistinguishable from those in already known complementation groups. Most derived t-mutations are viable, however. This paper summarizes data on the rate and types of variants produced by members of each of the six lethal complementation groups, and by semilethal alleles. It appears that particular complementation groups preferentially generate certain types of variants, and that in general, the pattern of variant production runs “uphill,” that is, to less abnormal states. The data are compatible with the hypothesis that t-mutations represent some extent of altered chromosome and that variants are produced by loss of abnormal material.

ECESSIVE lethal or semilethal mutations at the T/t locus in the mouse have the remarkable property of “mutating” to new forms of t-variants with

characteristics different from the parent allele. Although these lethal and semilethal mutations generally suppress genetic recombination in their vicinity, the generation of a new allele from a preexisting one is virtually always accompanied by an apparent crossover in the 8 centimorgan region distal to T marked by the mutation tufted, i.e., a t-mutation newly generated from another carries the marker gene of the opposite chromosome. Although other explanations are possible, the simplest one is a recombinational event.

The recessive lethal alleles fall into six different complementation groups. They and the semilethal alleles are pleiotropic. They interact with dominant T-mutations at the same locus to produce a tailless (IT/t) phenotype, they have homozygous effects on embryonic development, haploid and diploid effects on sperm function, and the above-mentioned suppression of genetic recombination (BENNETT 1975). Derivative alleles may be found in which any one or several

a L. C. DUNN was responsible for collecting and analyzing most of the data reported in this paper, but because of his death in March, 1974, he did not participate in writing the manuscript. His co-authors know that he would have made many improvements, but believe that in general he would have concurred with the arguments presented.

Genetics 83: 361-372 June, 1976.

362 D. B E N N E T T , L. C. D U N N A N D K. ARTZT

of these effects are altered or absent, but in no case has a complete revertant or wild-type allele been recovered.

Taken together, these facts have suggested that t-alleles may consist of abnormal segments of chromosome within which there are subunits, separable by some form of recombination, responsible for each of the effects mentioned above (LYON and MEREDITH 1964a, b, c). For this reason, we will hereafter use the term “haplotype” to refer to the segment of chromosome they occupy. A detailed analysis of the frequency and genetic behavior of the exceptional recombinants produced by the known lethal and semilethal haplotypes may provide an additional way of discriminating among them, as well as a potential means for mapping the separable genetic subunits responsible for their diverse effects.

This report gives data collected in our laboratories over the period 1961-1975 (updating the information in DUNN, BENNETT and BEASLEY 1962) , and presents evidence that t-haplotypes in different complementation groups differ not only in the rate at which they generate exceptional recombinants, but in the types of derivative chromosomes they produce.

MATERIALS A N D METHODS

Breeding System.

T Stocks: These are routinely maintained in a balanced system with matings between two tailless animals heterozygous for tufted (T t f / P +). In the absence of exceptional recombination, lines carrying lethal t-haplotypes breed true for tailless, non-tufted, since the T/T genotype as well as t n / t n is lethal in embryos. Occasional crossing over between T and tf is detected in these matings by the appearance of exceptional normal-tailed or tailless-tufted progeny. Genetic analysis routinely shows that these exceptions have a chromosome that carries both the tufted marker and a t-haplotype that is different from the parental one; their genotypes are, respectively, t” tf/P + and 2” tf/T t f . The expected reciprocal recombinant chromosome (T +) has not been detected in this type of cross. Clearly, this chromosome would be lethal in combination with a parental T tf gamete, and would not be phenotypically recognizable in combination with the other ( t m +) parental gamete in balanced crosses. However, animals from balanced stocks are frequently used in outcross experiments where a 7’ + chromosome should be revealed. No such chromosome has been detected, though, and our failure to detect it suggests that it may also be lethal when paired with the alternative parental gamete, t m +. The implications of this are discussed on page 369.

In one lethal complementation group, ( t 9 ) recombination between T and tf occurs with normal frequency, although some recombinant chromosomes nevertheless carry a derivative t-mutation. In this case all tailless-tufted offspring must be progeny-tested to determine whether they carry the parental ( 2 9 ) haplotype o r an altered chromosome (2” t f ) with a different t-haplotype. In tailless stocks that carry semilethal alleles, normal-tailed ( tn/ tm) offspring are expected, therefore recombinant chromosomes are detectable only when tailless-tufted offspring occur. For uniformity and to provide a known numerical base, all the data on frequency of exceptional recombination presented here are derived from the observation of these two recombinant phenotypes in the standard balanced type of mating. Many other kinds of matings are, of course, car- ried out, and occasional recombinants observed; where possible, these have been genetically analyzed and used to contribute information on characteristics of derived haplotypes, but they have not been included in calculations of rate of recombination since their numerical base is difi- cult to compute.

T- LOCUS M U T A T I O N S IN M O U S E 363

Analysis of Recombinants.

Exceptional normal-tailed (t" +/t" tf) animals are routinely crossed by T t f / f tf; the resulting progeny are scored at 28 days for homozygosity for tufted. Those that are phenotypically tufted are diagnosed as carrying the variant haplotype and are selected for inbreeding to produce tailless stock (T tf/t" tf) for further analysis. Tailless stocks carrying each recombinant are first bred inter se to determine whether the newly generated t-haplotype is viable or lethal in homozygous condition. If the haplotype behaves as a recessive lethal, it is tested for complementation with other known lethals (by crossing T t f / t z tf x T tf/t" + and assaying for normal tailed t" t f /P + progeny), and by that means, either assigned to a known category of t-lethal or designated as a new complementation group. In either case, affected homozygous embryos are studied histologically to supplement the genetic data. Whether the derived allele is lethal or viable, information is obtained on the following parameters:

a) Transmission ratio from male heterozygotes. Male heterozygotes (T/t") are crossed by normal (+/+) females and progeny scored for the ratio of short ( T / + ) and normal-tailed ( t" /+) phenotypes.

b) Recombination between T and tf. Male or female t" heterozygotes (usually T +/tz tf, sometimes T tf/t" +) are crossed to + tf/+ tf and the progeny scored with respect to both tail length and the tufted phenotype.

RESULTS

Cumulative data (including results from DUNN, BENNETT and BEASLEY 1962) for the rate at which exceptional chromosomes are generated, are shown in Table 1. Two things are immediately clear from this table. First of all, different complementation groups produce new haplotypes at rather different frequencies, which may vary by as much as a factor of four. Second, the rate of detection of exceptional chromosomes by tailless-tufted progeny is always less than the rate observed from normal-tailed progeny.

Table 2 lists the properties of derived variants that have been described since those reported in DUNN, BENNETT and BEASLEY (1962) , and Table 3 summarizes the characteristics of all the newly generated haplotypes that have been studied in our laboratories (including those summarized in DUNN, BENNETT and BEASLEY 1962). As can be seen from these tables, the great majority of derived haplotypes are viable, and of these, the majority have normal male transmission ratios and permit recombination in their vicinity. Only three of the six known complementation groups are represented among the few lethal mutations that have been extracted, and of these, four were in the t9 group, two in the tW1 series, and one representative of tZ2.

DISCUSSION

The observations presented here suggest that there is some pattern to the way in which lethal and semilethal t-mutations generate t-variants different from themselves. As Figure 1 shows, all lethal mutations are capable of conversion directly to viable haplotypes. Furthermore, all lethal haplotypes, with the exception of to, produce variants of the t9 type, (which is the most "normal" of the lethal t-haplotypes, having minimal effects on transmission ratio and permitting genetic recombination in its vicinity). In very general terms, the process of conversion to new lethal variants seems to be unidirectional and to run uphill;

364 D. B E N N E T T , L. C. D U N N A N D K. ARTZT

TABLE 1

Generation of t-mutations from known t-haplotypes

Scored at birth Scored at weaning Male - Rate of Rate of

Comple- trans- Tailless normal tailless Overall mentation mission Haplo- Normalcl non- Tailless tailed! tufted! rate

group ratio type Tailless tailed! tufted tufted! xi03 xi02 xi03

to 30 t9 .50

TOTALS

t'2 .75

TOTALS

t W' .95

TOTALS

tw5 .95

TOTALS

tW7S .95 tsemilethal .95

TOTALS

22,383 25! 334 0

1,320 2! 4,046 4!

233 0 502 l !

6,435 7!

7,253 19! 14,508 28! 21,761 47!

6,202 2! 1,815 l ! 6,497 15! 1,731 2!

628 0 1,526 2!

18,399 22!

10,571 18! 1,561 4!

432 0 961 l! 595 0 255 l! 358 0 623 3! 437 I! 359 0

16,152 28!

1,760 0 - - - - - - - _

4,649 2! - - - - - - _ - - - - -

1,074 I! 3,083 4! 4,157 5!

1,864 0

193 l! 430 0 88 0

375 0 2,956 l !

2,882 l ! 111 0 115 0 22 0 16 0

27 0 116 0

81 0 3,370 l!

405 0

- -

- -

_ -

1.12 0 1.51 0.99 0 1.99 1.09

2.61 1.93 2.16

0.32 0.55 2.30 1.15 0 1.31 1.19

1.70 2.56 0 1.04 0 0.39 0 4.79 2.28 0 1.73

0 2,449 2!(+4)$ - 1,392 3!(+1)$ -

340 0 (+ I )$ - 310 0 -

4,431 5!(+6)$ -

0.430 - - - - - - 0.93 1.30 1.20

0

5.00 0 0 0 0.34

0.35 0 0 0 0

0 0

0 0.30

0

-

-

-

- - - - -

1 .oo 0 1.51 0.99 0 1.99 1.09

2.40 1.82 2.00

0.32 0.55 2.38 0.92 0 1.05 1.08

1.41 2.39 0 1.02 0 0.39 0 4.04 2.28 0 1.48

0 0.82/2.44$ 2.15/2.87$

0 0/3.22s

1.11/2.44s

* Includes data for t 1 after 1953, by which time it was realized that t o and t1 stocks were

-f Includes data for tl prior to 1953, during which time the mutation known as t1 appears to

$ Includes t x tf chromosome recovered from normal-tailed parents.

identical because of a laboratory mix-up. (SILAGI 1962).

have been the same as the gene later designated tis. (SILAGI 1962).

Rate detected as tailless-tufted/Rate detected as tailless-tufted and as normal-tailed animals that transmitted a 1" tf! chromosome.

q ! denotes exceptional progeny carry variant t-haplotype.

T- LOCUS MUTATIONS IN MOUSE

TABLE 2

Characteristics of t-haplotypes deriued from known lethal and semi-lethal t-nutations*

365

Recombination T-tf

Haplotype of Male In males In females Derived Phenotype origin/comple- transmission ratio - haplo- of original mentation Homo- _ _ _ ~ % %

type exception group zygote nt Br %2: Rec./total ret.+ Rec./total rec.t

t z 3 t 3 1

t33

t 3 3

t 3 4

235

t36

t 3 7

238

t 3 9

140

t4'3:

t4z t 4 3

t 4 4

t 4 5

146

t 4 7

248

t w s 4

t w s 5

t w 4 2

t w 4 s

t w 4 4

t w 4 5

t W S 8

t W 4 0

t W 4 8

tW50

t w 5 1

t W 5 2

t w 5 s

p 5 4

t w 5 5

t w 5 7

t W 5 3

t W 5 6

t W 5 8

t w 6 0

t W 6 1

t W 6 2

tW6S

t W 6 4

t w 6 5 a

viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable viable

I45 425 25 50 72 41 80 80 50 88 102 46 54 176 23

229 430 35 118 107 52 55 48 53

102 506 17 68 295 19 94 251 27

49 46 52 27 25 52 72 163 31

_ - -

- - - - - - 6 33 -

156 270 37 220 315 41 184 503 27 186 514 27 301 380 44 216 185 54 133 135 50 210 243 46 311 306 50 242 408 37 38 80 32

138 136 50 lethal ( t 9 ) 368 212 viable 97 154 viable 168 266 viable 94 97 viable 50 53 viable 141 289 viable 117 294 viable 129 168 viable 148 181 viable 150 140 viable 197 187 viable 141 124 viable 65 130 viable 29 14

63 39 39 49 49 .33 28 43 .45 52 51 53 33 -

t W 6 5 b nt! tws"/tl" viable 64 30 .68

5/188 2.7

17/116 14.7 7/83 8.4

10/169 6.0

- -

- - - - 6/73 8.2 8/188 4.3

9/130 6.9 - - - - - - - - 0/14 - - - - - - - - - - -

14/218 6.4 8/254 3.1

14/249 5.6 8/43 - 3/44 -

10/182 5.5 3/199 1.5

11/206 5.3 - - 3/25 - 6/280 2.1 3/53 5.7

17/266 6.4 11/75 14.7 4/84 4.8

11/276 4.0 7/2& 3.4 3/98 3.1 5/108 4.6 8/142 5.6 7/68 10.3 1/44 - - - - - - -

366 D. BENNETT, L. C. DUNN AND K. ARTZT

TABLE 2-Continued

Recombination T-tf

Haplotype of Male In males In females Derived Phenotype origin/comple- transmission ratio haplo- of original mentation Homo- ______ % % type exception group zygote nt Br %t.t RecJtotal ret.+ Rec./total ref.+

tW66 tW67 tW68 tW69 tW70 tW76 tW77S tW78 tw79 tW82

tW83 tW84 tW85

nt! ot tf! -s nt! nt! nt! nt! 4 --D nt! tf! nt! ot tf!

viable viable viable viable viable viable viable viable viable viable viable viable viable

102 130 44 31 47 40 37 42 47 31 70 31 28 16 - 36 27 57 11 11 - 18 22 - 44 117 27

155 133 53 93 149 38

123 131 48 60 60 .50

* Alleles numbered through tZ8 and are reported in DUNN, BENNETT and BEASLEY 1962. This list includes all alleles derived from other known mutants in the course of breeding ex eri ments whether or not they arose in standard balanced lethal lines (see p. 00). It does not in8ud; t-mutants found existing in laboratory or wild populations of mice; data on these are reported separately (see BENNETT 1975 for review).

+Percentages are reported only for sample sizes of 50 o r more. $ Line lost before testing for other than viability. $ Allele did not originate in a balanced cross.

for example, the tW5 group is capable of generating lethals of most other classes, tw' and tIZ give rise only to t9, and t9 generates only viables, as does the semilethal class. It might have been expected that this pattern would have correlated with the stage at which these lethal mutations produce embryonic lethality. This is not the case, however; t 1 2 for example, being the earliest to act; tW5 intermediate, and tW1 the latest. It should be noted, moreover, that no complete revertants to wild type are ever detected.

TABLE 3

Characteristics* of haplotypes derived from the various T/t locus complementation groups

Derived viable haplotypes

Derived lethal haplotypes Transmission ratio Complementation group of origin Number Designation Number Noma1 Low

to 0 - 17 10 7 t9 0 - 4 4 0 t'" 2 t 4 , t9 18 11 7 t W' 1 twso 11 9 2 t W 5 5 t l a , ' I , tW18 t W 2 0 t W Z I t W 5 2 8 5 3 p e m i l e t h a l 0 - 7 5 2

* I t should be noted that the absolute numbers of haplotypes derived by recombination in each complementation group that are given in this table have different numerical bases; therefore the important comparisons are among the types of alleles produced by any one complementation group: e.g., lethal versus viable, o r low-ratio viable versus normal-ratio viable.

T- LOCUS MUTATIONS IN MOUSE 367

FIGURE 1 .-The pattern in which existing t-mutations generate new haplotypes.

In order to estimate the degree of confidence we may have concerning these data on the rates at which t-mutations produce exceptional recombinants and the types of new t-haplotypes produced, it is necessary to consider the ways in which exceptional recombinants are both detected and extracted. First, most recombinant chromosomes are detected in normal-tailed off spring of tailless stocks rather than as tailless-tufted exceptions. (Table 1). This is surprising in view of the fact that as a rule compounds of two1 different lethal t-mutations are not fully viable (DUNN, BENNETT and BEASLEY 1962). Therefore, it might be expected that the overall detection of recombinant chromosomes would occur most often in the tailless-tufted class, since the normal-tailed class will be compromised to the extent that it contains tZ-’/tz” compounds. A partial explanation for this discrep- ancy is that when recombination occurs in females, the resulting gamete has a high probability of being fertilized by a t-bearing sperm, rather than by a sperm carrying the T tf chromosome. This heightened probability, which ranges from 70-99 %, depending on the particular complementation group studied, is caused by the transmission ratio advantage of t-haplotypes in sperm. When a male produces a recombinant, of course, the resulting zygote has an equal chance of being genetically t2 tf/t” -I- or tx tf/T tf. It is simple to calculate that if both sexes

368 D. BENNETT, L. C . DUNN AND K. ARTZT

produce recombinants with equal frequency, parental haplotypes with male transmission ratios of about .70-.SO (e.g., to and tIB) should show a ratio of normal-tailed: tailless-tufted recombinants of somewhat less than 2, assuming full viability of the normal-tailed compounds. Under the same circumstances, however, that ratio should be about 3 from parental haplotypes such as tW1 and tW5 with very high transmission ratios of t (over .95). However, Table 1 shows that the ratio of normal-tailed vs. tailless-tufted exceptions is considerably higher, ranging from at least 2 to 6. Considering that the theoretical norm was based on full viability of normal-tailed compounds for two lethal mutations, which we know is not correct, the true deviation must be considerably greater. This suggests that recombinant chromosomes may be recovered predominantly from females, a factor that would tend to increase the number observed as normal-tailed exceptions. This may be only a reflection of the fact that crossing over is generally more frequent in females (DUNN and BENNETT 1967). Or it may mean that crossing over in the t-region actually does occur more often in females and thus provide a potential clue to understanding how this aberrant form of recombination occurs, An alternative but not mutually exclusive explanation is that sperm carrying newly generated haplotypes with low transmission ratios may fail to compete successfully for fertilization. In any case, many of the recombinational events that give rise to lethal variants must never be detected at all, because compounds of two lethal mutations are not fully viable. The viability of genotypes carrying two complementary haplotypes ( tZ-I/P2) relative to normal (+/+) varies from about 100% in the case of t9 combined with any other t-lethal, to about 10% for to/tlZ compounds (BENNETT, DUNN, ARTZT and DOOHER, in preparation). This means not only that derivative lethal mutations will he detected at a frequency that is less than that with which they occur, but also that lethal haplotypes arising from a given complementation group will have a probability of detection that depends on the properties of both the parent and the recombinant-derived haplotype. Also, since about half of the normal-tailed exceptions are male, and since male compounds ( tz-I /P2) of two lethal t-mutations are always sterile, only 50% of recombinant chromosomes carrying lethal t-haplotypes are potentially susceptible to analysis. The two factors of impaired viability and male sterility in normal-tailed exceptions no doubt contribute, but to an inestimable extent, not only to the overall rate at which exceptions are detected, but also to the relatively low proportions in which lethal uersus viable haplotypes are recovered from recombinants.

Therefore, the rate at which known complementation groups appear to generate exceptions cannot be taken as a measure of the true frequency, nor can exceptions that come to analysis be relied on as truly representative of the types of alleles that can potentially be generated. Nevertheless, it seems clear that particular complementation groups preferentially generate only certain types of variants. The most obvious example of this is the t9 group, which produces exceptions only rarely, and then only viable haplotypes. Since mutations in the t9 group show virtually complete complementation with all other t-lethals, other things being equal, the frequency of recovery of lethals from this group is


expected to be the highest of any. Thus the fact that no lethals at all have been recovered from t9 is no doubt significant. Coupled with this are the observations that to also has a relatively low rate of generation of viable haplotypes, and that all these viable chromosomes have normal transmission ratios. Taken together, these data suggest that the t9 chromosome is relatively uncomplicated, carrying either a single lethal factor or one of restricted size. The relative simplicity of the tQ-haplotype is probably also reflected in the high frequency with which it is produced by recombination in other lethal t-mutations, 5 of the 8 lethal recombinant-derived variants reported here being of the t9 type. Although part of this is undoubtedly due to the good complementing properties of the t9 haplotype, part of it must also be due to a propensity on the part of other lethal t-mutations to revert to the more normal t9 state.

Other t-haplotypes also appear to show preferred patterns of change; some support for this is seen in Figure 1, but the difficulties of detection and assay mentioned above, as well as the small numbers involved, makes these suggestions less than definitive.

Different complementation groups produce recombinants at rates that are clearly different (Table 1) , although for reasons given, the statistical significance of these differences is virtually impossible to calculate. In any event, there is no easily discernible correlation between the rate at which new haplotypes are produced, and the types of variants that are derived. For example, to and tQ produce only viable haplotypes and do so with relatively low frequency, but semilethal mutations, which also produce only viables, have the highest rate of all. Likewise, tW5, which stands out from the other complementation groups both by virtue of the fact that it produces many more lethal derivatives than any other group, and because it itself has never been derived by recombination, has a rate of recombination that is neither particularly high nor particularly low. This suggests that the rate at which t-mutations generate new haplotypes does not depend solely on such simple factors as the size of the chromosomal lesion or quantitative variation in the number of mutant elements they contain.

Nevertheless, the data given here, as well as that presented by others (LYON and MEREDITH 1964a, b, c) make it reasonable to assume at least that lethal and semilethal t-mutations occupy some extent of chromosome. A part of their mutated region must be unilocal with T since recombination never gives rise to a normal chromosome, and an additional segment must exist distal to T since it can be dissected by recombination, and yield haplotypes with new properties. Therefore, we might more accurately diagram the genotype of the animals from which these data were obtained as T tf/t" -I- where * * * * indicates the

possibility that in lethal or semilethal t-haplotypes much of the region between the loci of T and tf is altered. With any model it is necessary to take account of the fact that all of the recombinant haplotypes that have been detected in this study have been of the t" tf variety; and that we have never recovered the

expected reciprocal crossover chromosome T (t") f. As already mentioned, this

* * * * * * *

I***

* * *

370 D. BENNETT, L. C. DUNN A N D K. ARTZT

chromosome would be lethal when paired with a parental chromosome carrying T tf. Furthermore, if the factor or factors responsible for the specific type of leth- z y produced by a given t-haplotype were always in the more distal segment, re-

+) with their allele of origin. This in fact was true with respect to the only derived haplotype (PI8) that has been recovered from the reciprocal type of recombinant. In this case, the putative distal segment was recovered because of a different mating system; the chromosome alternate to the t-haplotype was of the + tf rather than the - T tf type. This recombinant chromosome was derived f r o m x e ts-haplotype (complementation group to) and proved to cany a lethal factor identical to te, which could be mapped to a region very close to tufted, i.e., in the distal segment of the t-region. Furthermore, it had lost not only the capacity to interact with T to produce taillessness, but also the properties of recombination suppression and transmission ratio distortion that were associated with t s (LYON and MEREDITH 1964a). These facts and other data led LYON and MEREDITH (1964b) to propose that t-mutations consisted of different parts, separable by recombination, responsible for their various effects such as lethality, suppression of recombination, and male transmission ratio. They suggested also that t-mutations might consist of a functional change over some length of chromosome, perhaps resulting in abnormal heterochromatization, and further that haplotypes in different complementation groups might be of different lengths. At that time they speculatively proposed that the known t-mutations could be arranged in order of decreasing length as Iollows: tW5, tZU1, t12, to, t9.

The additional data we have obtained permits us to rank these mutations in the same relative order, on the basis of a model that takes into account only the lethal factors that comprise the genotypes of tW5, tW1, tI2 and t9. Referring again to Table 3, where it is seen that the pattern in which new haplotypes are

combinants of this sort would also be lethal when paired (T (t") +/tn * * * * * * * * * *

generated can be represented as follows: tt05 + tW1 tW1 t9 t ' 2 + t9

t ' 2 t" t" t 9

t" we can assume that the homozygous

t9 + t"

lethal phenotype of tW5 is due to four _ _ separable mutant segments arranged in order distal to-the site of T itself (and the site of the t-factor responsible for the interaction that leads to the phenotype of taillessness in all T/t heterozygotes). The logical order of these segments in the tW5 allele is

-T interaction factor (=t"?) t9 tlS tW' tW5 1-1-1-1

Recombination that separates the "tW5" lethal factor from more proximal segments could thus produce the tW1 haplotype. Likewise, the other alleles under discussion can be diagrammed as follows:

T-interaction tW1: factor (=t"?) , t9 t12 tW1

i---l-i-l


T-interaction tI2: factor (=t"?) t9 tIg

T-interaction t9: factor (=t"?) t9

-- 1-1-1

- -I-I The semilethal haplotypes and to cannot be fitted into the model at present because appropriate data do not exist, nor can we readily incorporate into it the elements responsible for transmission ratio distortion and recombination suppression.

This model explains how these t-haplotypes can be derived from one another in the pattern observed. It suggests further that the ultimate phenotypic effects of t-mutations are due to complex interactions among the separable elements of which they are composed which result in the whole being more than the simple sum of its parts. There are several reasons for this statement. First of all, each of the different mutant haplotypes shows complementation with others; yet if homozygous lethality were dependent on simple homozygosity for any one of the separable lethal factors, we would have to expect that tW5 would not complement any other allele. Since this is not the case, it must mean that the properties of specific lethal factors are somehow conditioned by the presence of still other lethal factors in their chromosomal region. Second, if these various lethal factors operated independently of one another in their effects on embryonic development, we would expect to find that the linear sequence of lethal factors in our model correlated with the time of embryonic death in homozygotes, with the distal one the longest and worst and therefore capable of giving rise by recombination only to lethal factors less deleterious. That is, if the relationship were a simple one, the sequence expected would be: tW1, t9, tW5, tl2, since the time when embryonic abnormalities are first observed in homozygotes for these mutations is, respectively, about 9 days, 8 days, 7 days, and 3 days. Since, on the basis of our model, homozygotes for twl for example, are thought to contain the lethal factors for both tl2 and t9, it is difficult to understand how they develop normally through the lethal periods of both t12 and t9 unless the function of the mutations in this region represents a non-arithmetic sum of their parts. Finally, serological exam- ination of antigens on sperm specified by three of these haplotypes (t'", tW' and tW5) did not indicate any cross-reactivity among them (YANAGISAWA et al. 1974). This again suggests that the overall complex of mutant lethal factors in a given haplotype yields a product that is not composed of independent non-interactive units, but is unique to the particular complex of lethal factors involved.

Thk work was supported by AEC Contract AT (11-1) 3115 to Columbia University, ERDA Contract AT (11-1) 2497 to Cornell University Medical College, and NSF Grant GB 33804X to Cornell. We are very grateful also for the invaluable help of J. B. COOKINGHAM, E. SCHERMEK- HORN, L. HAMBURGER, and C. CALO.

LITERATURE CITED

BENNETT, D., 1975

Dunn, L. C., D. BENNETT and A. B. BEASLEY, 1962 of a complex gene. Genetics 47: 285-303.

The T-locus of the mouse: A review. Cell. 6: 441-454.

Mutation and recombination in the vicinity

3 72 D. BENNETT, L. C. DUNN A N D K. ARTZT

DUNN, L. C. and D. BENNETT, 1967 Sex differences in recomhination of linked genes in animals. Genetical Research 9: 211-220.

LYON, M. F. and R. MEREDITH, 1364a Investigations of the nature of t-alleles in the mouse. I. Genetic analysis of a series of mutants derived from a lethal allele. Heredity 19: 301-312. -- -, Investigations of the nature of t-alleles in the mouse. 11. Genetic analysis of a.a unusual mutant allele and its derivatives. Heredlty 19: 313-325. -, 1964.c Inves- tigations of the nature of t-alleles in the mouse. 111. Short tests of some further mutant alleles. Heredity 19: 327-330.

SILAGI, S., 1962 A genetical and embryological study of partial complementation between lethal alleles at the T-locus of the house mouse. Devel. Biol. 5 : 35-67.

YANAGISAWA, K., D. BENNETT, E. A. BOYSE, L. C. DUNN and A. DIMEO, 1974 Serological iden- tification of sperm antigens specified by lethal t-alleles in the mouse. Immunogenetics 1: 57-67.

1964b

Corresponding editor: A. CHOVNICK

t/t-locus in the mouse - genetics

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