Hybrid Breakdown in Cichlid Fish

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.

Testing the hybrid superiority hypothesis in crested and marbled newts

“…Since T. cristatus – T. marmoratus hybrids are largely sterile and introgression is near‐absent, hybridization between these two species appears to be an evolutionary dead‐end (Arntzen et al, , ). This supports the theory of hybrid breakdown in the F 2 , similar to the study of Stelkens et al () on cichlid fish hybrids. At the other side of the coin, heterosis appears to be absent when there is gene flow, as in the hybrid population of T. macedonicus and T. ivanbureschi (Arntzen et al, ).…”

“…Studies from a wide range of taxa have shown a negative relationship between genetic compatibility and the divergence time of hybridizing genomes (Stelkens et al, ), since selection acts against hybrid individuals, probably due to disadvantageous combinations of co‐adapted gene complexes derived from either parental species, the so‐called Dobzhansky–Muller model of hybrid incompatibility (e.g., Parsons, Olson, & Braun, ; Wu & Ting, ). Theory predicts the main breakdown of fitness to happen after the F 1 hybrid generation, when heterosis subsides and recessive allelic incompatibilities are increasingly unmasked and cause hybrid sterility.…”

“…Therefore, two opposing mechanisms are in place: hybrid incompatibility acts to avert ongoing hybridization while heterosis favors the maintenance of hybrids. Incompatible alleles at different loci, inherited from the parental species, may not be harmful in F 1 but can be deleterious when homozygous, a condition which should be more frequent in higher generation hybrids, potentially causing hybrid breakdown in F 2 (Stelkens, Schmid, & Seehausen, ). Since hybrid zones are dynamic, interspecific variation in life‐history traits is an important parameter in understanding them (e.g., Scribner, ).…”

Mechanical Transgressive Segregation and the Rapid Origin of Trophic Novelty

“…The incredibly recent timeframe during which the several hundred species within Lake Malawi have diverged has provided comparatively little time for intrinsic reproductive barriers to arise and reduce hybrid viability154950. Therefore, hybridization and transgression in Lake Malawi cichlids could readily occur between two species with highly divergent trophic morphologies because hybridization between Lake Malawi cichlids is very common1114.…”

What Role Has Hybridization Played in the Replacement of Native Roanoke Bass with Invasive Rock Bass?

“…Third, disruption of co‐adapted nDNA gene complexes can cause hybrid breakdown, particularly following recombination in second‐generation hybrids and beyond (e.g., Rogers and Bernatchez ; Stelkins et al. ). The detection of inferred F 2 × pure backcrosses in our study indicates that at least some second‐generation hybrids were fertile and at least some third‐generation hybrids were viable.…”