Mating system, auxosporulation, species taxonomy and evidence for homoploid evolution in Amphora (Bacillariophyta)

Abstract

Mann D.G. and Poulíčková A. 2010. Mating system, auxosporulation, species taxonomy and evidence for homoploid evolution in Amphora (Bacillariophyta). Phycologia 49: 183–201. DOI: 10.2216/09-08.1Cytological characteristics of the mitotic cycle, sexual reproduction and auxospore formation are described for three members of the type group of Amphora: A. ovalis, A. copulata and A. minutissima. All have a single lobed ventral chloroplast, nuclei with granular heterochromatin and a single nucleolus (unusually large in A. ovalis). Amphora copulata is heterothallic, with two mating types that do not differ in gamete behaviour. All three species reproduce allogamously via fusion of two rearranged gametes per gametangium to produce elongate zygotes enclosed within the volume delimited by the gametangial thecae. Pairing is strictly via the ventral sides of the cells, with tight apposition of the cells, and expansion of the auxospores is strictly perpendicular to the gametangium long axes. Comparisons are made with other Amphora species, particularly A. proteus (also belonging to the subgenus Amphora) and A. arcus. Meiosis is described in detail in A. copulata, revealing no movement of the nucleus away from the dorsal position it occupies during the mitotic cycle, a synizetic knot in early prophase, no cytokinesis accompanying meiosis II, and rapid degeneration of the supernumerary nuclei in the gametic cells; the usual two acytokinetic mitoses accompany formation of the initial epivalve (which lacks a functional raphe) and hypovalve. The potential for formation of autopolyploids is demonstrated in A. copulata. Nevertheless, chromosome counts of 2n = 34 in A. copulata, 2n = 28 or 30 in A. ovalis and an undetermined but not significantly lower number in A. minutissima, suggest that homoploid evolution is more likely than polyploid. In mixed sexualized populations of the species, no interbreeding occurs, supporting separation of minutissima from copulata. Measurements of initial cell sizes, coupled with slight differences in valve width and pattern, suggest that A. copulata may be a species complex.