Heterosis and epistasis for teat number and fluctuating asymmetry in crosses between Jiaxing and Iberian pigs.

In the framework of Wright's view of evolution, long-separate breeds of domestic animals could establish different adaptive epistatic genetic complexes that could be destroyed in crossbred animals by recombination. The objective of this study was to evaluate heterosis and recombination effects in a crossing experiment involving two distinct European and Asian breeds (Iberian and Jiaxing) in the F1 and two successive backcrosses to the Iberian line. Teat number (TN) was recorded in the right and left sides of piglets and analysed by fitting a mixed linear model including the Dickerson's crossbreeding parameters. TN in pigs is a discontinuous and often canalised trait presenting bilateral symmetry. The minor differences between sides make this trait a good candidate to evaluate fluctuating asymmetry (FA) and developmental instability. For TN, the posterior means and standard deviations (SD) of the heritability and of the relative contribution of common litter environmental effect to variance were 0.248 (0.028) and 0.057 (0.019), respectively. The respective values of the difference between breeds, heterosis and recombination effect were 9.990 (0.411), -0.506 (0.196) and 0.684 (0.232). For FA, the posterior means and SDs of the heritability and of the relative contribution of common litter environmental effect to variance were 0.023 (0.005) and 0.014 (0.005), respectively. Another significant genetic effect was a recombination effect of 0.773 (0.117). These results confirm that the rupture by recombination of coadapted genomes decreases developmental stability in domestic pigs.