Systematics and evolution of Ludwigia section Dantia (Onagraceae).

摘要

Ludwigia sect. Dantia is a polyploid complex (x=8) of five species that are distributed mainly in the Coastal Plain of the southeastern United States, although two species, L. palustris and L. repens, have much broader ranges in North America and have become naturalized in Eurasia and elsewhere. All five species occur in wet habitats and often intermix; because the species generally lack well-developed pre- or post-zygotic barriers to hybridization, several of them commonly form natural hybrid populations. One particularly widespread hybrid involves L. palustris x L. brevipes, known in the literature as L. x lacustris. Hybrids of L. arcuata x L. repens are common in Florida and, because they superficially resemble L. brevipes, have led to considerable confusion about the presence there of L. brevipes. Ludwigia sect. Dantia comprises one diploid, two tetraploid, and two hexaploid species. The genomic formula of the diploid L. palustris was designated as AA, and by means of a crossing program the genomes of other species were determined based on ability to form bivalents at meiosis in the hybrids. Of the two tetraploid species, only L. spathulata (AADD) has a genome of the extant diploid; L. arcuata (BBCC) is sharply distinct from L. spathulata in both morphology and genomic complements. The two hexaploid species, L. brevipes and L. repens, share the genomic formula AABBCC and appear to have been derived independently through hybridization between the diploid L. palustris (AA) and the tetraploid L. arcuata (BBCC). The substantial morphological differences between the two hexaploids may be related to the direction of the original crosses, since maternal influences are important in the group, as well as to the exact parental populations involved, subsequent selection history, and possibly backcrossing. Alternative evolutionary schemes are also discussed..