VARIATION IN SEED CHARACTERS IN NEMOPHILA MENZIESII : EVIDENCE OF A GENETIC BASIS FOR MATERNAL EFFECT

Abstract

A growing body of evidence indicates that phenotypic selection on juvenile traits of both plants and animals may be considerable. Because juvenile traits are typically subject to maternal effects and often have low heritabilities, adaptive responses to natural selection on these traits may seem unlikely. To determine the potential for evolutionary response to selection on juvenile traits of Nemophila menziesii (Hydrophyllaceae), we conducted two quantitative genetic studies. A reciprocal factorial cross, involving 16 parents and 1960 progeny, demonstrated a significant maternal component of variance in seed mass and additive genetic component of variance in germination time. This experiment also suggested that interaction between parents, though small, provides highly significant contributions to the variance of both traits. Such a parental interaction could arise by diverse mechanisms, including dependence of nuclear gene expression on cytoplasmic genotype, but the design of this experiment could not distinguish this from other possible causes, such as effects on progeny phenotype of interaction between the environmental conditions of both parents. The second experiment, spanning three generations with over 11,000 observations, was designed for investigation of the additive genetic variance in maternal effect, assessment of paternal effects, as well as further partitioning of the parental interaction identified in the reciprocal factorial experiment. It yielded no consistent evidence of paternal effects on seed mass, nor of parental interactions. Our inference of such interaction effects from the first experiment was evidently an artifact of failing to account for the substantial variance among fruits within crosses. The maternal effect was found to have a large additive genetic component, accounting for at least 20% of the variation in individual seed mass. This result suggests that there is appreciable potential for response to selection on seed mass through evolution of the maternal effect. We discuss aspects that may nevertheless limit response to individual selection on seed mass, including trade-offs between the size of individual seeds and germination time and between the number of seeds a maternal plant can mature and their mean size.

The Evolutionary Genetics of an Adaptive Maternal Effect: Egg Size Plasticity in a Seed Beetle

“…In the Apache Junction population, there were large sire and dam (nested within sire) effects on both the size of eggs than the treatment of seed size in recent studies by some plant evolutionary biologists (e.g., Byers et al 1997). Because we treat eggs as maternal characters, the effect of a mother's phenotype or environment on the size of eggs that her daughters lay is a maternal effect on egg size (estimated as V M in the analysis described above).…”

“…Here, we describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in S. limbatus and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid on A. greggii and the size of eggs laid on C. floridum (the cross-environment genetic correlation; Fry 1992). Few other studies have quantitatively assessed genetic variation in maternal effects and the genetic constraints on their evolution, such that the potential for maternal effects to respond to natural selection remains largely speculative (Roach and Wulff 1987;Riska 1991;Byers et al 1997;Shaw and Byers 1998).…”

“…plants (e.g., Byers et al 1997). The heritability of egg size ranged between 0.217 and 0.908 for S. limbatus, varying between populations (significantly higher in the Apache Junction than in the Del Rio population) and hosts (higher on A. greggii than on C. floridum).…”

“…We now also know that maternal effects themselves are often genetically variable and can thus respond to selection (e.g., Wade 1998, papers in Mousseau andFox 1998). However, few studies have quantitatively assessed genetic variation in maternal effects, par-ticularly those that are presumed to have evolved as adaptations (Roach and Wulff 1987;Riska 1991;Byers et al 1997;Shaw and Byers 1998). Because maternal effects can confound our interpretation of within and among population variation in morphological and life-history traits (Sinervo 1991;Fox 1994aFox , 1997aFox et al 1995), researchers generally consider them in experiments only to avoid overestimating additive genetic variances and avoid confusing environmentally based maternal effects with genetic variation (Mazer 1989;Shaw and Byers 1998).…”

Seed fates in crop–wild hybrid sunflower: crop allele and maternal effects

“…For example, if seeds from the crop parent lack seed dormancy and those of the wild parent have extensive dormancy, all crop-wild F 1 cross types might be hypothesized to have intermediate levels, depending on the dominance levels of the alleles involved. However, seed traits, including dormancy, can be disproportionately affected by the maternal parent, that is, experience genetic maternal effects (Byers et al 1997). These can be due to matrilineal inheritance of cytoplasm, organelles, and chloroplasts, unequal endosperm contribution, and maternal tissues surrounding the seed during and after development (Roach and Wulff 1987).…”