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Replicated hybrid zones of Xiphophorus swordtails along an elevational gradient


Natural hybrid zones provide opportunities to study a range of evolutionary phenomena
from speciation to the genetic basis of fitness-related traits. We show that widespread
hybridization has occurred between two neo-tropical stream fishes with partial
reproductive isolation. Phylogenetic analyses of mitochondrial sequence data showed
that the swordtail fish Xiphophorus birchmanni is monophyletic and that X. malinche is
part of an independent monophyletic clade with other species. Using informative single
nucleotide polymorphisms in one mitochondrial and three nuclear intron loci, we
genotyped 776 specimens collected from twenty-three sites along seven separate stream
reaches. Hybrid zones occurred in replicated fashion in all stream reaches along a
gradient from high to low elevation. Genotyping revealed substantial variation in
parental and hybrid frequencies among localities. Tests of FIS and linkage disequilibrium
(LD) revealed generally low FIS and LD except in five populations where both
parental species and hybrids were found suggesting incomplete reproductive isolation.
In these locations, heterozygote deficiency and LD were high, which suggests either
selection against early generation hybrids or assortative mating. These data lay the
foundation to study the adaptive basis of the replicated hybrid zone structure and for
future integration of behaviour and genetics to determine the processes that lead to the
population genetic patterns observed in these hybrid zones.