darwinisme i evolució al segle xxi base genètica de l’aïllament postzigòtic
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Darwinisme i evolució al segle XXI Base 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 - PowerPoint PPT PresentationTRANSCRIPT
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