The Biosystematics of Ludwigia Sect. Microcarpium (Onagraceae)

Open Access
Publisher Website
Google Scholar
Abstract
Ludwigia sect. Microcarpium is a polyploid complex of 14 species distributed primarily in the southeastern United States. Several of the species are variable and taxonomically difficult; the boundaries of many are blurred by intermediate forms. Relationships among the species are often reticulate and in some cases difficult to specify precisely. Data from the study of meiosis in artificial and natural hybrids were used to analyze relationships among the species. Among diploid (n = 8) species, L. linearis and L. linifolia are similar in morphology but differ chromosomally by one reciprocal translocation. F1 hybrids between them are vigorous, have 47-48% stainable pollen, and produce moderate quantities of viable seeds. Another diploid, L. microcarpa, is morphologically and genetically distinct from L. linearis and L. linifolia. Hybrids between L. microcarpa and either other diploid are vigorous but produce only 6% stainable pollen and no viable seeds. They formed mostly univalents at meiotic metaphase I, with up to three sometimes heteromorphic bivalents. The fourth diploid, L. stricta, has not been studied biosystematically. The eight morphologically distinct tetraploid (n = 16) taxa can be crossed in any combination, producing fertile offspring with complete association of their 16 bivalents. No extant diploids are believed to have been involved directly in forming the tetraploids of sect. Microcarpium. Ludwigia alata, a hexaploid (n = 24), is morphologically similar to the tetraploids, and chromosome pairing in experimental hybrids suggests that it originated after hybridization between a tetraploid and L. microcarpa or populations ancestral to that species. Two chromosome numbers are present in the L. curtissii complex (L. simpsonii, n = 24; L. curtissii, n = 32). The hexaploid complement of L. simpsonii appears to include three different diploid genomes, one of which is identical with that of L. microcarpa. Present data are not sufficient to determine if the diploid L. linearis-L. linifolia lineage was involved in parentage of the hexaploid L. simpsonii, but morphological evidence suggests it was not. It is highly probable, however, that the octoploid L. curtissii was derived after hybridization between the diploid L. linearis-L. linifolia lineage and the hexaploid L. simpsonii.