Ecological genetics of seed germination regulation in Bromus tectorum L. I. Phenotypic variance among and within populations

Regulation of seed germination phenology is an important aspect of the life history strategy of invading annual plant species. In the obligately selfi ng winter annual grass Bromus tectorum, seeds are at least conditionally do rmant at dispersal in early summer and lose dormancy through dry-afterripen ing. Patterns of germination response at dispersal vary among populations a nd sometimes across years within populations. To assess the relative contri bution of genotype and maturation environment to this variation, we grew pr ogeny of ten parental lines from each of six contrasting populations in a c ommon greenhouse environment. We then tested the germination responses of r ecently harvested seeds of the putative full-sib progeny at five incubation temperatures. Significant germination response differences among populatio ns were observed in greenhouse cultivation, and major differences among ful l-sib families were evident for some populations and traits. Among-populati on variation accounted for over 90% of the variance in each trait, while wi thin-family variance accounted for 1% or less. Germination responses of gre enhouse-grown progeny were positively correlated with the responses of wild -collected seeds, but there was a tendency for lowered dormancy at higher i ncubation temperatures. This tendency was more marked in populations from c old desert, foothill, and plains habitats, suggesting a genotype-maturation environment interaction. Differences among populations in the amount of am ong-family variance were more evident at lower incubation temperatures, whi le among-family variance was more uniformly low at summer incubation temper atures. Populations from predictable extreme environments (subalpine meadow and warm desert margin) showed significantly less variation among families than populations from less predictable cold desert, foothill, and plains e nvironments. Low among-family variance was not specifically associated with small population size or marginality of habitat, as small marginal populat ions from unpredictable environments showed variance as high as that of lar ge populations. In populations with high among-family variance for germinat ion traits, germination responses tended to be correlated across incubation temperatures, making it possible to characterize families in terms of thei r general dormancy status. The results indicate that seed germination regul ation in this species is probably under strong genetic control, and that ha bitats with temporally varying selection are occupied by populations that t end to be more polymorphic in terms of their germination response patterns.