RESUMO
Seed germination strategies vary dramatically among species but relatively little is known about how germination traits correlate with other elements of plant strategy systems. Understanding drivers of germination strategy is critical to our understanding of the evolutionary biology of plant reproduction.We present a novel assessment of seed germination strategies focussing on Australian alpine species as a case study. We describe the distribution of germination strategies and ask whether these are correlated with, or form an independent axis to, other plant functional traits. Our approach to describing germination strategy mimicked realistic temperatures that seeds experience in situ following dispersal. Strategies were subsequently assigned using an objective clustering approach. We hypothesized that two main strategies would emerge, involving dormant or non-dormant seeds, and that while these strategies would be correlated with seed traits (e.g., mass or endospermy) they would be largely independent of vegetative traits when analysed in a phylogenetically structured manner.Across all species, three germination strategies emerged. The majority of species postponed germination until after a period of cold, winter-like temperatures indicating physiological and/or morphological dormancy mechanisms. Other species exhibited immediate germination at temperatures representative of those at dispersal. Interestingly, seeds of an additional 13 species "staggered" germination over time. Germination strategies were generally conserved within families. Across a broad range of ecological traits only seed mass and endospermy showed any correlation with germination strategy when phylogenetic relatedness was accounted for; vegetative traits showed no significant correlations with germination strategy. The results indicate that germination traits correlate with other aspects of seed ecology but form an independent axis relative to vegetative traits.
RESUMO
BACKGROUND AND AIMS: When species cohesion is maintained despite ongoing natural hybridization, many questions are raised about the evolutionary processes operating in the species complex. This study examined the extensive natural hybridization between the Australian native shrubs Lomatia myricoides and L. silaifolia (Proteaceae). These species exhibit striking differences in morphology and ecological preferences, exceeding those found in most studies of hybridization to date. METHODS: Nuclear microsatellite markers (nSSRs), genotyping methods and morphometric analyses were used to uncover patterns of hybridization and the role of gene flow in morphological differentiation between sympatric species. KEY RESULTS: The complexity of hybridization patterns differed markedly between sites, however, signals of introgression were present at all sites. One site provided evidence of a large hybrid swarm and the likely presence of multiple hybrid generations and backcrosses, another site a handful of early generational hybrids and a third site only traces of admixture from a past hybridization event. The presence of cryptic hybrids and a pattern of morphological bimodality amongst hybrids often disguised the extent of underlying genetic admixture. CONCLUSIONS: Distinct parental habitats and phenotypes are expected to form barriers that contribute to the rapid reversion of hybrid populations to their parental character state, due to limited opportunities for hybrid/intermediate advantage. Furthermore, strong genomic filters may facilitate continued gene flow between species without the danger of assimilation. Stochastic fire events facilitate temporal phenological isolation between species and may partly explain the bi-directional and site-specific patterns of hybridization observed. Furthermore, the findings suggest that F1 hybrids are rare, and backcrosses may occur rapidly following these initial hybridization events.
