Pennisetum section Brevivalvula in West Africa : morphological and genetic variation in an agamic species complex

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Abstract

Section Brevivalvula is one of five sections in the large tropical grass genus Pennisetum . It belongs to the tertiary genepool of P. glaucum (L.) R. Br., pearl millet, and consists of six morphological species: P. atrichum Stapf & Hubb., P. hordeoides (Lam.) Steud., P. pedicellatum Trin., P. polystachion (L.) Schult., P. setosum (Swartz) L. Rich. and P. subangustum (Schum.) Stapf & Hubb. P. setosum and P. atrichum are perennials, the other species are annual.Four euploid (2x, 4x, 5x, and 6x with x = 9) chromosome levels are known in the section. Tetraploid cytotypes are dominant in all taxa, except P. setosum , which is predominantly hexaploid. Diploids and pentaploids are rare.Some of the polyploid cytotypes of P. pedicellatum , P. hordeoides , and P. polystachion are at least partly facultative apomicts, while diploid P. polystachion and P. subangustum are reproducing sexually.A morphological analysis showed that all taxa intergrade with at least one other taxon. P. pedicellatum , with one - or more - pedicelled spikelets in large, fluffy involucres, has differentiated from the group with a single sessile spikelet per involucrum, but intermediate plants with one almost sessile spikelet exist as well. The group with a single sessile spikelet shows a gradient from slender plants with scabrous involucral bristles on narrow inflorescences ( P . hordeoides ), to slender plants with hairy involucral bristles on narrow inflorescences ( P.subangustum ), to large plants with hairy involucral bristles on large inflorescences ( P.polystachion ), to large perennial plants with hairy involucral bristles, on large yellowish inflorescences ( P. setosum ). P. atrichum is grouped near P. hordeoides , because of its scabrous involucral bristles, but it is also perennial and rather large.Isozyme electrophoresis of 635 plants resulted in 146 different 5-locus genotypes from combinations of 26 alleles. More than 85% of the samples of different species are connected by patterns of identical genotypes. When the proportions of 20 alleles are compared between species, ploidy levels or geographical areas, significant differences are found everywhere. The results also indicate that the samples share the same gene pool.These results indicate that the complex is in an active state of evolution. Speciation occurs at a low rate because of sexuality in the diploids as well as facultative apomixis in part of the polyploids, which cause hybridization events among the taxa, obscuring species boundaries. Speciation can become successful when a morphotype finds a specific niche for itself. This is clearly the case for P. pedicellatum , which is better adapted to drier climats than the other taxa. Because of the hybridizations, a fully unambiguous description of the other species of Pennisetum section Brevivalvula is impossible.

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