Auxosporulation, mating system, and reproductive isolation in Neidium (Bacillariophyta)

Abstract

D.G. Mann and V.A. Chepurnov. 2005. Auxosporulation, mating system, and reproductive isolation in Neidium (Bacillariophyta). Phycologia 44: 335–350.

Three allogamous Neidium demes, referrable to N. ampliatum sensu lato, coexist without intergradation in Blackford Pond, Edinburgh, and some other lakes. A slight change in valve shape has occurred in one deme during the last 20 years. Morphological deme traits persist in healthy clonal cultures, but with time or during senescence, aberrant morphologies and sizes of cell can be produced that have no parallel in nature. Simplification of valve outline occurs as cells get smaller, but the initial cells also have a simplified morphology. The rostrate apices of some N. ampliatum demes develop rapidly after auxosporulation, during the first divisions of the initial cells. The ‘major’ and ‘minor’ demes of N. ampliatum are heterothallic, although some inbreeding occurs. Very rarely, mixed pairs of ‘major’ × ‘minor’ are formed, but hybrid auxospores are apparently never produced, so the demes are reproductively isolated. The ‘maternal’ gametangium has a nongenetic influence on initial cell size; however, the effect is slight and control of initial cell size is very well buffered to variation in gametangium size, so the concept of ‘cardinal points’ is valid for this species complex. The characteristics of sexual reproduction in clones and seminatural populations (including the effective, though incomplete, suppression of triplets and larger groups during copulation) prompt hypotheses about pairing mechanisms, in particular that a chemoattractant is involved. Preferential polyandry in Sellaphora and theoretical considerations indicate that the chemoattractant is produced by only one mating type. Size selection of mates in the ‘minor’ deme probably reflects progressively easier and more rapid sexualization as cells become smaller.

ACKNOWLEDGMENTS

We are grateful to INTAS for two grants (93-3605 and 93-3605-ext) that helped us establish our programme of research on the mating systems of diatoms and to the Royal Society for an equipment grant to D.G. Mann, allowing purchase of a Polyvar photomicroscope. V.A. Chepurnov is currently supported by grant BOF98/GOA.03 from the University of Gent, Belgium, and by the Fund for Scientific Research – Flanders (Belgium) FKFO-project G.0292.00.