transgression

Stelkens RB, Schmid C, Selz O & Seehausen O 2009 Phenotypic novelty in experimental hybrids is predicted by the genetic distance between species of cichlid fish. BMC Evol Biol 9:283.

  • transgression is most commonly due to complementary gene action or epistasis, which become more frequent at larger genetic distances
  • 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
  • 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
  • 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
  • another potential outcome of hybridization that may facilitate adaptive diversification into new directions is the occurrence of qualitatively or quantitatively novel phenotypes referred to as transgressive segregation
  • it is most often caused by the action of complementary genes between QTL loci that carry alleles of opposite signs in the parents but sum up to larger or smaller trait values compared to the parents when combined in a hybrid genom
  • one interesting prediction emerging from this, especially put forward by Rieseberg et al. [25], is that the amount of transgression should increase as a function of the genetic distance between the parental lines
  • transgression and phenotypic differentiation have been suggested to be inversely correlated
  • phenotypically similar species produce more transgressive hybrid offspring than phenotypically dissimilar parents
  • because large phenotypic differences between two species may often result from divergent directional selection, a process expected to eventually lead to the fixation of alleles with the same sign across all QTL within a species, and mostly opposite signs between the species
  • if the parents show rather similar phenotypes, despite considerable genetic distance, this indicates the action of stabilizing selection
  • the genetic basis for transgression is then more likely
  • because stabilizing selection leads to alternating fixation of alleles with negative and positive trait values, and the sequence of fixation of alleles with either sign at different QTLs will by chance be different between isolated populations
  • in agreement with this prediction, a study on transgression in hybrids between two cichlid fish species revealed novel phenotypes only in traits with a selection history other than consistent directional selection
  • we predicted to observe an increase of transgression with increasing genetic crossing distance in both F1 and F2 hybrids
  • in F1 hybrids, the increase may be predicted to be less steep than in F2 hybrids because (besides epistatic interactions) only dominant genetic effects can contribute to complementary gene action
  • in F2 hybrids, additive genetic effects also contribute