Résumé
Genetic introgression from a resident species into an invading close
relative can result from repeated hybridisation along the invasion front
and/or allele surfing on the expansion wave. Cases where the phenomenon is
massive and systematic, such as for hares (genus Lepus) in Iberia, would
be best explained by recurrent hybridisation but this is difficult to
prove since the donor populations are generally extinct. In the Pyrenean
foothills, Lepus europaeus presumably replaced Lepus granatensis recently
and the present species border is parallel to the direction of invasion,
so that populations of L. granatensis in the contact zone represent
proxies of existing variation before the invasion. Among three pairs of
populations sampled across this border, we find less differentiation of
mtDNA across than along it, as predicted under recurrent hybridisation at
the invasion front. Using autosomal microsatellite loci and X and Y-linked
diagnostic loci, we show that admixture across the border is quasi-absent,
making it unlikely that lack of interspecific mtDNA differentiation result
from ongoing gene flow. Furthermore, we find that the local species ranges
are climatically contrasted, making it also unlikely that ongoing
ecology-driven movement of the contact account for mtDNA introgression.
The lack of mtDNA differentiation across the boundary is mostly due to
sharing of mtDNA from a boreal species currently extinct in Iberia (Lepus
timidus) whose mitochondria have thus remained in place since the last
deglaciation despite successive invasions by two other species. Home
loving mitochondria thus witness past species distribution rather than
ongoing exchanges across stabilised contact zones.