MATING SYSTEM, SEXUAL REPRODUCTION, AND AUXOSPORULATION IN THE ANOMALOUS RAPHID DIATOM EUNOTIA (BACILLARIOPHYTA)¹

Abstract

The diatom genus Eunotia is unusual among raphid diatoms in having a raphe system consisting of two short slits that are not integrated into the primary pattern center. This and other characteristics, particularly the presence of rimoportulae, are consistent with the hypothesis that Eunotia is a basal lineage within the raphid group. We studied auxosporulation in E. bilunaris (Ehrenberg) Mills and E. tropica Hustedt for comparison with other raphid pennate diatoms and with araphid pennates; E. bilunaris was studied in parental and F1 generations. Like araphid pennates, E. bilunaris and E. tropica are heterothallic. Clones of the same mating type did not interact sexually, and intraclonal sexual reproduction was absent or very rare. Clones retained the same sex throughout the life cycle, as shown by experiments using abrupt size reduction to produce clones of similar age but different size and using subclones derived from a single initial cell within six mitotic generations. Unlike in araphid pennate diatoms, in the Eunotia species the gametes are not visibly or behaviorally differentiated. Gametogenesis is merogenous, because the gametangium formed a supernumerary cell as well as a single gametic cell, both undergoing meiosis II to form a surviving functional nucleus and a nucleus that quickly degenerated. Plasmogamy is via papillae that grew out toward each other from the ends of the gametangia to create a copulation canal. After plasmogamy, the gametes moves bodily into the copulation canal, producing an elongate zygote, which expands to form a curved sausage-like auxospore.

Congruence of morphological, reproductive and ITS rDNA sequence data in some AustralasianEunotia bilunaris(Bacillariophyta)

“…Valve structure and auxospore formation documented here were broadly as described by Mann et al (2003), and Australasian E. bilunaris, like Scottish E. bilunaris, is heterothallic. However, there are several discrepancies that are probably significant (see below).…”

“…bilunaris' clones studied by Mann et al (2003). The Scottish and Australasian clones are all heterothallic and have similar sexual reproduction (pole-to-pole pairing, papilla formation, formation of a residual cell, etc.).…”

“…Auxospore structure Mann et al (2003) did not study the auxospores of their E. bilunaris clones by SEM and did not detect a perizonium; nor did they examine release of the initial cells from the auxospore, which might have revealed disrupted perizonia. The only previous study containing extra detail of the auxospore casing and initial cell in Eunotia is the LM-based report by Geitler (1951), but Idei (in Mann et al, 2003) has reported the existence of a perizonium of transverse bands in E. tropica Hustedt.…”

“…The only previous study containing extra detail of the auxospore casing and initial cell in Eunotia is the LM-based report by Geitler (1951), but Idei (in Mann et al, 2003) has reported the existence of a perizonium of transverse bands in E. tropica Hustedt.…”

“…Vanormelingen@UGent.be form of the raphe endings. Thorough taxonomic revision is therefore needed and it is likely that the species should be split (Krammer & LangeBertalot, 1991;Mann et al, 2003).…”