Non-recombining sex chromosomes are expected to undergo evolutionary decay,
ending up genetically degenerated, as has happened in birds and mammals. Why are
then sex chromosomes so often homomorphic in cold-blooded vertebrates? One
possible explanation is a high rate of turnover events, replacing master
sex-determining genes by new ones on other chromosomes. An alternative is that
X-Y similarity is maintained by occasional recombination events, occurring in
sex-reversed XY females. Based on mitochondrial and nuclear gene sequences, we
estimated the divergence times between European tree frogs (Hyla
arborea, H. intermedia, and H.
molleri) to the upper Miocene, about 5.4–7.1 million years
ago. Sibship analyses of microsatellite polymorphisms revealed that all three
species have the same pair of sex chromosomes, with complete absence of X-Y
recombination in males. Despite this, sequences of sex-linked loci show no
divergence between the X and Y chromosomes. In the phylogeny, the X and Y
alleles cluster according to species, not in groups of gametologs. We conclude
that sex-chromosome homomorphy in these tree frogs does not result from a recent
turnover but is maintained over evolutionary timescales by occasional X-Y
recombination. Seemingly young sex chromosomes may thus carry old-established
sex-determining genes, a result at odds with the view that sex chromosomes
necessarily decay until they are replaced. This raises intriguing perspectives
regarding the evolutionary dynamics of sexually antagonistic genes and the
mechanisms that control X-Y recombination.
Background
Hybridogenesis can represent the first stage towards hybrid speciation where the hybrid taxon eventually weans off its parental species. In hybridogenetic water frogs, the hybrid
Pelophylax
kl.
esculentus
(genomes RL) usually eliminates one genome from its germline and relies on its parental species
P. lessonae
(genomes LL) or
P. ridibundus
(genomes RR) to perpetuate in so-called L-E and R-E systems. But not exclusively: some all-hybrid populations (E-E system) bypass the need for their parental species and fulfill their sexual cycle via triploid hybrid frogs. Genetic surveys are essential to understand the great diversity of these hybridogenetic dynamics and their evolution. Here we conducted such study using RAD-sequencing on
Pelophylax
from southern Switzerland (Ticino), a geographically-isolated region featuring different assemblages of parental
P. lessonae
and hybrid
P.
kl.
esculentus
.
Results
We found two types of hybridogenetic systems in Ticino: an L-E system in northern populations and a presumably all-hybrid E-E system in the closely-related southern populations, where
P. lessonae
was not detected. In the latter, we did not find evidence for triploid individuals from the population genomic data, but identified a few
P. ridibundus
(RR) as offspring from interhybrid crosses (LR × LR).
Conclusions
Assuming
P. lessonae
is truly absent from southern Ticino, the putative maintenance of all-hybrid populations without triploid individuals would require an unusual lability of genome elimination, namely that
P.
kl.
esculentus
from both sexes are capable of producing gametes with either L or R genomes. This could be achieved by the co-existence of L- and R- eliminating lineages or by “hybrid amphigamy”, i. e. males and females producing sperm and eggs among which both genomes are represented. These hypotheses imply that polyploidy is not the exclusive evolutionary pathway for hybrids to become reproductively independent, and challenge the classical view that hybridogenetic taxa are necessarily sexual parasites.
Electronic supplementary material
The online version of this article (10.1186/s12862-019-1482-4) contains supplementary material, which is available to authorized users.
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