Phenotypic novelty in experimental hybrids is predicted by the genetic distance between species of cichlid fish

Abstract

BackgroundTransgressive segregation describes the occurrence of novel phenotypes in hybrids with extreme trait values not observed in either parental species. A previously experimentally untested prediction is that the amount of transgression increases with the genetic distance between hybridizing species. This follows from QTL studies suggesting that transgression is most commonly due to complementary gene action or epistasis, which become more frequent at larger genetic distances. This is because the number of QTLs fixed for alleles with opposing signs in different species should increase with time since speciation provided that speciation is not driven by disruptive selection. We measured the amount of transgression occurring in hybrids of cichlid fish bred from species pairs with gradually increasing genetic distances and varying phenotypic similarity. Transgression in multi-trait shape phenotypes was quantified using landmark-based geometric morphometric methods.ResultsWe found that genetic distance explained 52% and 78% of the variation in transgression frequency in F1 and F2 hybrids, respectively. Confirming theoretical predictions, transgression when measured in F2 hybrids, increased linearly with genetic distance between hybridizing species. Phenotypic similarity of species on the other hand was not related to the amount of transgression.ConclusionThe commonness and ease with which novel phenotypes are produced in cichlid hybrids between unrelated species has important implications for the interaction of hybridization with adaptation and speciation. Hybridization may generate new genotypes with adaptive potential that did not reside as standing genetic variation in either parental population, potentially enhancing a population's responsiveness to selection. Our results make it conceivable that hybridization contributed to the rapid rates of phenotypic evolution in the large and rapid adaptive radiations of haplochromine cichlids.

Artificial interspecific backcrosses between the hybrid of female Crassostrea hongkongensis × male C. gigas and the two parental species

“…The ratio of the H, HG and G locus types was approximately equal to 1:2:1 following the genetic segregation of Mendelian law. This result differed from several reports on the abnormal genetic patterns of fish F 2 hybrids analyzed using molecular methods (Liu, 2003;Liu et al, 1998;Stelkens et al, 2009). Collectively, the backcross progeny and F 2 hybrids showed a degree of genetic recombination and segregation despite using only one nuclear marker.…”

Anthropogenic impacts on the contextual morphological diversification and adaptation of Nile tilapia (Oreochromis niloticus, L. 1758) in East Africa

“…In addition, Tilapiine species (Oreochromis esculentus and Oreochromis niloticus) have also been reported to hybridize under experimental conditions (Welcomme 1966). Other GM studies regarding African cichlids (haplochromines) in lake Tanganyika reported pronounced shape variations in the F1 hybrids and concluded that hybridization among cichlids contribute significantly to morphological divergent (Stelkens et al 2009;Kerschbaumer and Sturmbauer 2011). The outstanding morph divergent exhibited by Victoria and Kyoga populations might therefore be an expression of introgression between Nile tilapia and its native relatives which might be one possible explanation why the native Tilapiines vanished.…”

Phenotypic traits of primate hybrids: Recognizing admixture in the fossil record

“…This results in the occurrence of extreme trait values not observed in either parental taxon, including novel phenotypes, 27 ultimately increasing phenotypic variation in hybrid populations. Transgressive segregation has been shown to increase with increasing genetic distance between parental species 27 due to a rising number of fixed alleles with opposite effects. 28 Transgression in important traits is one means by which hybrids can invade and succeed in niches that are distinct from their parental species.…”