Population genetic structure and clonal diversity of Allium oleraceum (Amaryllidaceae), a polyploid geophyte with common asexual but variable sexual reproduction

Abstract

Clonal plants are, on average, considered to be as genetically diverse as nonclonal plants. However, the behaviour of clonal plants ranges between multiclonality and uniclonality, depending on environmental conditions and life history traits. Allozyme electrophoresis of band phenotypes was used to examine the genetic structure of 13 cytotype-uniform and 17 cytotype-mixed populations of polyploid Allium oleraceum (2n = 4x, 5x, 6x), a clonal bulbous geophyte that has been considered seed-sterile and completely reliant on vegetative reproduction through aerial bulbils and daughter bulbs. The genetic structure was dominated by low within-population variation whereas genetic differentiation was high among populations. Eighty-five distinct multilocus genotypes were found among 756 sampled individuals, but populations generally exhibited a low level of clonal diversity. Tetra- and pentaploids showed twofold higher total and within-population diversity, but also had more genetically differentiated populations in comparison with hexaploids. Tetraploids formed two separate groups in the cluster analysis, and this finding most likely suggests their different origin. Pentaploids were grouped in a separate cluster and frequently intermixed with tetra- and hexaploids sampled at cytotype-mixed sites. Such a pattern suggests gene flow between cytotypes. Most hexaploids were genetically similar and clustered separately from the other cytotypes, suggesting their similar origin and absence of gene flow to and from other cytotypes. Identical band phenotypes found in coexisting cytotypes within certain mixed-ploidy populations might indicate in situ neopolyploidization. Collectively, the pattern of genetic structure and diversity observed in A. oleraceum is typical of clonal plants with the dominance of vegetative offspring and scarce recruitment of sexual offspring. The low and spatially unstructured genetic variation observed in hexaploids, in contrast with higher and spatially structured genetic variation in tetra- and pentaploids, seems to be related to different levels of sexual fertility, ecological amplitude and colonization abilities of the cytotypes. It provides evidence for the existence of both primary and secondary contact zones of cytotypes in A. oleraceum.