Restricted X chromosome introgression and support for Haldane’s rule in hybridizing damselflies

Contemporary hybrid zones act as natural laboratories for the
investigation of species boundaries and allow to shed light on the little
understood roles of sex chromosomes in species divergence. Sex chromosomes
are considered to function as a hotspot of genetic divergence between
species; indicated by less genomic introgression compared to autosomes
during hybridisation. Moreover, they are thought to contribute to
Haldane’s rule which states that hybrids of the heterogametic sex are more
likely to be inviable or sterile. To test these hypotheses, we used
contemporary hybrid zones of Ischnura elegans, a damselfly species that
has been expanding its range into the northern and western regions of
Spain, leading to chronic hybridization with its sister species Ischnura
graellsii. We analysed genome-wide SNPs in the Spanish I. elegans and I.
graellsii hybrid zone and found (i) that the X chromosome shows less
genomic introgression compared to autosomes and (ii) that males are
underrepresented among admixed individuals as predicted by Haldane’s rule.
This is the first study in Odonata that suggests a role of the X
chromosome in reproductive isolation. Moreover, our data adds to the few
studies on species with X0 sex determination system and contradicts the
hypothesis that the absence of a Y chromosome causes exceptions to
Haldane’s rule.