1 Darwinisme i evolució al segle XXI Base genètica de l’aïllament postzigòtic

2 The Origin of Species (1859)

3  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

4 REALITY OF SPECIES  Concordance between “folk” and “scientific” species  Statistical identification of clusters

5 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

6 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

7 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)

8 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

9 Classification of reproductive isolating barriers  Postzygotic isolating barriers (hybrid sterility and inviability)  A. Extrinsic  Ecological inviability  Behavioral inviability  B. Intrinsic  Hybrid inviability  Hybrid sterility

10 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)

11 GroupAsymmetric phenotype Number obeying hybridizations a Haldane’s rule Heterogametic males DrosophilaSterility114112 Inviability1713 MammalsSterility25 Inviability11 Heterogametic females LepidopteraSterility11 Inviability3429 BirdsSterility2321 Inviability30 a Hybridizations in which only one hybrid sex is sterile or inviable

12 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

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15  The parental D. simulans stock was homozygous for the recessive mutants f 2 ; 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 (2 5 ) possible backcross classes were chosen for analysis of male fertility

16  F 1 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

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

19 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

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

21 Dobzhansky-Muller model  Consider the case in which both loci are autosomal  Allele A 1 from the first species is incompatible with B 1 from the second species  Because A 1 and B 1 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

22 Dobzhansky-Muller model Now consider the case where one locus (A) is X- linked and the other (B) is autosomal ♀ A 1 A 1 B 2 B 2 x ♂ A 2 YB 1 B 1 ↓ ♀♀ A 1 A 2 B 1 B 2 + ♂♂ A 1 YB 1 B 2 Hybrid females will be inviable only if A 1 and B 1 are fairly dominant. If A 1 is recessive and B 1 dominant, hybrid females remain fit. But under the same conditions hybrid males are dead

23 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

24 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

25 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

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27 Inviabilitat dels híbrids a Drosophila ♀ D. simulans x ♂ D. teissieri ↓ ♀♀ híbrides Els mascles híbrids moren durant la primera o segona fase larvària

28 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

29 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

30 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

31 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

32 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

33 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

34 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

35 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

36 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

37 Metodologia experimental  Retroencreuaments/anàlisi F 2  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

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