Darwinisme i evolució al segle XXIBase genètica de l’aïllament postzigòtic

The Origin of Species (1859)

The origin of species had much more to say about change within species that about the origin of new species

Darwin saw the origin of species as a direct consequence of the struggle between individuals for ecological elbow room

REALITY OF SPECIES

Concordance between “folk” and “scientific” species

Statistical identification of clusters

There is a remarkable coincidence between folk species and Linnaean species

Tribesmen of the Arfak Mountains of New Guinea had 136 vernacular names for the 137 Linnaean species of birds they encountered (Mayr, 1963)

Of the exceptions that do exist, most involve under- rather than overdifferentiatioon

Statistical identification of clusters

Folk taxonomy is a form of cluster analysis, but one can use more sophisticated statistical tools to look for clustering

Discriminant-function analysis, principal coordinates analysis

Concepte biològic d’espècie

Les espècies són grups de poblacions naturals formades per individus que es poden encreuar entre ells i que estan aïllats reproductivament d’altres grups

E. Mayr (1942). “Systematics and the origin of species (pàg. 120)

Classification of reproductive isolating barriers

I. Premating isolating barriers Behavioral isolation Ecological isolation Mechanical isolation Mating system isolation

II. Postmating, prezygotic isolating barriers Copulatory behavioral

isolation Gametic isolation

Classification of reproductive isolating barriers

Postzygotic isolating barriers (hybrid sterility and inviability) A. Extrinsic

Ecological inviability

Behavioral inviability

B. Intrinsic Hybrid inviability Hybrid sterility

La regla de Haldane

J. B. S Haldane. 1922

Quan a la descendència de dues races o espècies animals, un dels sexes és rar, no apareix o és estèril, es tracte del sexe heterogamètic (XY o ZW)

Group Asymmetric phenotype

Number obeying

hybridizationsa

Haldane’s rule

Heterogametic males

Drosophila Sterility 114 112

Inviability 17 13

Mammals Sterility 25 25

Inviability 1 1

Heterogametic females

Lepidoptera Sterility 11 11

Inviability 34 29

Birds Sterility 23 21

Inviability 30 30aHybridizations in which only one hybrid sex is sterile or inviable

Hybrid sterility in Drosophila

Dobzhansky pioneered the use of genetic markers to study hybrid sterility between the sibling species D. pseudoobscura and D. persimilis

This method, effectively the same as QTL mapping, is still being used by modern researchers

The parental D. simulans stock was homozygous for the recessive mutants f2; nt pm; st, e, with forked-2 on the X chromosome, net and plum on the two arms of the second chromosome and scarlet and ebony on the two arms of the third chromosome

Fourteen of the 32 (25) possible backcross classes were chosen for analysis of male fertility

F1 hybrid males are completely sterile. This sterility is due to failed spermatogenesis and not testicular atrophy, since the hybrid males have normal-sized testes but no visible sperm. Sterile males were those with either no sperm or wholly nonmotile sperm

The fertility of males in a genotypic class was defined as the proportion of males in that class having motile sperm

Effect of the X chromosome

The effect of the X chromosome can be seen by comparing genotypes 1 vs 2 or 13 vs 14

Results of this analysis

This analysis shows that all three major chromosomes have significant effects on male fertility

Similarly, each of the four autosomal arms has a significant effect on male fertility

The most dramatic reduction of fertility is caused by the X chromosome

Theodosius Dobzhansky (1900-1975)-Hermann J. Muller (1890-1967)

Dobzhansky-Muller model

Consider the case in which both loci are autosomal

Allele A1 from the first species is incompatible with B1 from the second species

Because A1 and B1 occur as heterozygotes, hybrids will be inviable only if both factors are fairly dominant

We have no reason to expect differences in the fitness of hybrid males vs. females

Dobzhansky-Muller model

Now consider the case where one locus (A) is X-linked and the other (B) is autosomal

♀ A1A1B2B2 x ♂ A2YB1B1

↓♀♀ A1A2B1B2 + ♂♂ A1YB1B2

Hybrid females will be inviable only if A1 and B1 are fairly dominant. If A1 is recessive and B1 dominant, hybrid females remain fit. But under the same conditions hybrid males are dead

The DOMINANCE theory

Hybrid males are affected by all X-linked genes involved in genic incompatibilities, dominant and recessive, whereas hybrid females are affected only by that subset of genes that are fairly dominant

So long as some fraction of the genes causing hybrid problems are recessive, hybrid males (heterogametic) should fare worse than hybrid females (homogametic), giving rise to Haldane’s rule

Note that the dominance theory hinges on alleles that act recessively in hybrids

The DOMINANCE theory

The simplest prediction of the dominance theory was noted by Coyne (1985): if hybrid males are sterile or inviable because they express recessive X-linked genes causing hybrid problems, then hybrid females that are homozygous for the same X should also be sterile or inviable

Genetic analyses support the dominance theory explanation for inviability

Use attached-X D.simulans stock to test the fertility of interspecific female hybrids having both of their X chromosomes from D. simulans but two species-specific sets

Inviabilitat dels híbrids a Drosophila

♀ D. simulans x ♂ D. teissieri

↓

♀♀ híbrides

Els mascles híbrids moren durant la primera o segona fase larvària

Resultats dels encreuaments entre espècies

Es disposa d’una soca de D. simulans (C(1)RM) amb els cromosomes X units

♀♀ ♂♂

♀ D. simulans Fla. City x ♂ D. teissieri Congo 51 0

♀ D. simulans yw x ♂ D. teissieri Congo 200 0

♀ D. simulans (C(1)RM) x ♂ D. teissieri Congo 0 0

Genetic analyses have ruled out the dominance theory explanation for sterility

Use attached-X D.simulans stock to test the fertility of interspecific female hybrids having both of their X chromosomes from D. simulans but two species-specific sets

Crosses:♀ D. simulans (attached-X) x ♂ D. mauritiana♀ D. simulans (attached-X) x ♂ D. sechellia

Female hybrids with two D. simulans chromosomes suffer no loss of fertility compared with the controls

The faster-male theory

Are there more genic incompatibilites affecting male than female hybrids?

The faster-male theory makes a simple prediction: chromosome regions moved from one species into another should include hybrid male steriles more often than hybrid female steriles

The prediction has been verified

Spermatogenesis might be an inherently sensitive process that is easily perturbed in hybrids

Sexual selection might cause faster evolution of male-than female expressed genes, since males are involved in male-female, as well as male-male interaction. These more diverged male genes would be more likely to cause problems in hybrids

Caveats

The theory has limited applicability: Faster-male evolution cannot explain Haldane’s rule for hybrid inviability - it is a theory of hybrid sterility

The theory cannot explain Haldane’s rule for sterility in taxa in which females are heterogametic

Quants gens determinen l’aïllament reproductor? Mayr (1963): “Species differences…

seem to be controlled by a large number of genetic factors with small individual effects. The genetic basis of the isolating mechanisms, in particular, seem to consist largely of such genes”

Malgrat això, la primera dissecció genètica d’un aïllament reproductor, en Crepis, implicava una incompatibilitat entre dos gens

Quin és el nombre de gens que ens interessa?

Cal precisar si el que interessa és el nombre de gens que ORIGINALMENT van produir l’aïllament reproductor o bé el nombre de gens que ACTUALMENT determinen l’aïllament entre les dues espècies

Opinions actuals

A. Base poligènica La identitat dels gens implicats no és rellevant. El

que compte és el seu nombre. La introducció de DNA no codificant d’una altra espècie pot provocar l’aïllament postzigòtic si la quantitat introduïda és suficient

La identitat dels gens implicats és important B. Pocs gens, o bé base poligènica peró amb

gens “majors” El nombre total de gens implicats depèn, en part,

de la potencia estadística per detectar efectes petits. El que compta és si una fracció important de les diferències interespecífiques són degudes a pocs gens

Cal distingir entre inviabilitat i esterilitat

Cada vegada es disposa de més proves que demostren que l’esterilitat dels híbrids està determinada per més gens que no pas la inviabilitat dels híbrids

Metodologia experimental

Retroencreuaments/anàlisi F2

Anàlisi de l’introgressió: retroencreuaments continuats amb l’espècie parental. Obtenció d’homozigots per les regions introgressades i anàlisi de la fitness

Anàlisi de deficiències