Oospore variation in three species of Chara (Charales, Chlorophyta)

Abstract

Members of the Characeae produce a resistant, thick-walled zygote called the oospore. Oospore characters have been used in taxonomic treatments of species of the two most diverse genera in the family, Nitella C.A. Agardh and Chara Linnaeus. However, the taxonomic value of oospore characters depends upon their morphological constancy within species. In order to determine the amount of oospore variation within species and populations of Chara, oospores of Chara australis R. Brown, C. fibrosa C.A. Agardh ex Bruzelius sensu Wood (1962), and C. muelleri A. Braun were examined using scanning electron microscopy. Chara australis, a dioecious perennial species, displayed a large amount of intrapopulation variation in the length, width, and number of striae of its oospores, but the ultrastructure of the oospore wall was consistent within and among populations. Chara muelleri, a monoecious species of temporary habitats, exhibited less within-population variation, and the ultrastructure of the oospore wall was consistently minutely granulate. Oospores of charophytes in temporary habitats may be subject to natural selection for size, a factor that could contribute to the comparative uniformity of C. muelleri oospores. Charophytes in permanent habitats may be less dependent on oospores for persistence and so oospore size may not be as consistently selected. In contrast to these two species, C. fibrosa sensu Wood (1962) exhibited a large range of oospore variation among populations. Wood’s delineation of C. fibrosa contains both monoecious and dioecious entities and populations with different chromosome numbers. The variation in oospore morphology in C. fibrosa underscores the need for revision of this species complex.