PARENTAL EFFECTS ON PROGENY PHENOTYPE IN PLANTS - DISTINGUISHING GENETIC AND ENVIRONMENTAL CAUSES

The experimental measurement of additive genetic variation in plant po pulations is complicated by the potential for non-Mendelian inheritanc e. Maternal influences on progeny phenotype resulting from the cytopla smic inheritance of plastids or RNA transcripts and effects of the mat ernal environment have consequently been the focus of much research. T o exclude or to control for these sources of variation, breeding desig ns (e.g., cross-factored, nested, or diallel) in which genetically unr elated pollen donors are mated to maternal genotypes have been adopted . Using these designs, some empirical studies have detected statistica lly significant differences among pollen donors in the mean performanc e of their pollen (the mature male gametophytes) or in the mean phenot ype of their progeny. These statistical effects of pollen-donor identi ty on pollen performance or progeny phenotype have frequently been int erpreted as evidence for additive genetic variance among pollen donors , although patrilineal cytoplasmic inheritance or effects of the pater nal environment on pollen performance or gene expression are recognize d as alternative explanations. We note that environment-specific selec tion among developing gametophytes-in which the environment experience d by developing pollen grains (or ovules) provides a selective force c ausing the differential survival of gametophyte genotypes (analagous t o meiotic drive)-is an additional process that may cause genetically b ased paternal (or maternal) effects on gametophyte performance. If gen es selected during this process are expressed in the sporophyte (postf ertilization), this process could also influence the phenotype of the diploid progeny. Here, we review the potential causes of statistically significant differences in mean phenotype among the gametophytes or p rogeny of maternal (seed-bearing) or paternal (pollen-donating) parent al plants. We suggest an experimental approach that permits the detect ion or elimination of selection among developing gametophytes as one s uch cause. Specifically, the replication of homozygous parental genoty pes within and across environments allows the detection and measuremen t of paternal and maternal environmentally induced effects on gametoph yte or offspring phenotype, while eliminating meiotic drive as a sourc e of the phenotypic variation.