Abstract
Studies from a wide diversity of taxa have shown a negative relationship between genetic compatibility and the divergence time of hybridizing genomes. Theory predicts the main breakdown of fitness to happen after the F1 hybrid generation, when heterosis subsides and recessive allelic (Dobzhansky-Muller) incompatibilities are increasingly unmasked. We measured the fitness of F2 hybrids of African haplochromine cichlid fish bred from species pairs spanning several thousand to several million years divergence time. F2 hybrids consistently showed the lowest viability compared to F1 hybrids and non-hybrid crosses (crosses within the grandparental species), in agreement with hybrid breakdown. Especially the short- and long-term survival (2 weeks to 6 months) of F2 hybrids was significantly reduced. Overall, F2 hybrids showed a fitness reduction of 21% compared to F1 hybrids, and a reduction of 43% compared to the grandparental, non-hybrid crosses. We further observed a decrease of F2 hybrid viability with the genetic distance between grandparental lineages, suggesting an important role for negative epistatic interactions in cichlid fish postzygotic isolation. The estimated time window for successful production of F2 hybrids resulting from our data is consistent with the estimated divergence time between the multiple ancestral lineages that presumably hybridized in three major adaptive radiations of African cichlids.
Highlights
The loss of genetic compatibility is one of the causes of reproductive isolation in classical models of speciation [1]
We measured the fitness of F2 hybrids of African haplochromine cichlid fish bred from species pairs spanning several thousand to several million years divergence time
The estimated time window for successful production of F2 hybrids resulting from our data is consistent with the estimated divergence time between the multiple ancestral lineages that presumably hybridized in three major adaptive radiations of African cichlids
Summary
The loss of genetic compatibility is one of the causes of reproductive isolation in classical models of speciation [1]. In the D-M model, alleles with neutral or beneficial effects in one population reduce offspring fitness when recombined in a hybrid genome with alleles originating at different loci in a different population [2,3,4]. These negative epistatic interactions can cause hybrid sterility and reproductive isolation [1, 5, 6]. Genetic incompatibilities are expected to accumulate the longer two taxa diverge independently from each other. This process, which typically extends over hundreds of thousands to PLOS ONE | DOI:10.1371/journal.pone.0127207 May 21, 2015
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