Seed dispersal provides a template upon which the demographic processes of plant life cycles operate and, most importantly, seed dispersal determines the spatial distribution of genetic variation. Here, we review recent studies on seed dispersal by water (hydrochory) in species of Hibiscus to elucidate the potential ecological and evolutionary consequences of hydrochory at different spatiotemporal scales. We consider seed dispersal related processes that determine the distribution and dynamics of genetic variation, that is, the spatial shuffling effect of the seed bank that modifies genetic make-up within a population, the formation of a genetic reservoir within a metapopulation, and lineage dispersions and speciation that may produce patterns in plant phylogeography. Seed movement patterns within ecological time and space are likely to leave non-random patterns in long-term plant evolution and phylogeny.

Seed dispersal is a major determinant of the spatial genetic structure of plant populations. In this study, we evaluated the role of distinct hydrologic regimes in determining the spatial genetic structure of the seed bank of the wetland plant Hibiscus moscheutos. We analyzed seeds in surface soil samples collected in the autumn and the following spring by determining their allozyme genotypes and estimated the pattern in seed movements during flooding. We selected study sites in nontidal and tidal wetlands with different flooding regimes. One nontidal site had no flooding, while the second nontidal site was inundated for most of the year. One tidal wetland site flooded with almost every tide, and a second tidal site was inundated at moderate frequency. Genetic makeup of the seed bank at the nonflooded site changed little between seasons. Secondary seed dispersal altered absolute allele frequencies at the other three sites, with the greatest change occurring at the two tidally influenced sites. This study demonstrates that secondary hydrochory influences the genetic composition of the seed bank and that hydrologic conditions play an important role in determining the local patterns in seed movements.

The intra-generic relationships of the Australian genus Calotis, with various chromosome base numbers from x = 8 to x = 4, were examined by the comparison of nucleotide sequences of the complete ITS region of nuclear rDNA and of the matK gene of chloroplast DNA. Within a monophyletic Calotis, four lineages were identified. Reconstruction of ancestral states suggests that the chromosome base number for Calotis is x = 8. Dysploidal reductions in chromosome base number from x = 8 to x = 7 and from x = 8 to x = 5 or 4 have occurred independently at least three times. Lower base numbers of x = 7, 5, and 4 are found predominantly in the arid and semi-arid zone species of Central and Western Australia. Total karyotypic length (genome size) is greater in perennials than in annuals within the genus Calotis. The elaborated pappus and surface structures of cypsela, and life form of species seem to be homoplasous with multiple origins in the evolutionary history of the lineage.

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