Maternal and paternal contributions to the fitness of hybrids between red and white mulberry (Morus, Moraceae)

The fitness of hybrids depends on the genetic disparity between parental taxa and the magnitude of their nuclear and nonnuclear contributions. To estimate the role of non-nuclear effects, we crossed red (R), white (W) and hybrid (H) mulberry in all combinations and compared the magnitude of maternal and paternal effects on offspring fitness (seed set, germination, survival and aboveground biomass) in a greenhouse environment. Variation in offspring fitness was determined largely by the identity of the maternal parent; specifically, progeny with white mothers had the highest cumulative fitness. As fathers, red, white, and hybrid mulberry had no effect on fitness, and maternal x paternal interactions were significant only for survival. Individual cross-types differed significantly for all fitness components except seed set. Offspring from hybrid crosses (W x R, H x R, H x W) often differed from at least one of the within-parent crosses (W x W, R x R) as well as from other hybrid crosses, although their fitness values never exceeded the most fit parent. Reciprocal crosses differed in only two of 15 possible parental combinations: W x H (cumulative fitness) and W x R (aboveground biomass). Overall, the strong asymmetry in magnitude of maternal and paternal effects suggests that fitness of hybrid mulberry is governed largely by non-nuclear, parental effects.