Dioecious Reproduction in Achnanthes longipes (Bacillariophyta)

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The benthic diatom Achnanthes longipes exhibits dioecious as well as the previously reported monoecious reproduction. In both cases, two paired maternal cells isogamously produce two auxospores. Nonmonoecious clones are also capable of vegetative cell expansion. Regardless of sex, they periodically produce single auxospores by monoecious reproduction. Closely related progeny also exhibit splitting of regressive characters such as bisexuality of the clone, formation of two adhesive mucilaginous stalks by the cells, and increasing stalk length as the culture density increases.

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Sexual reproduction is an essential phase in the life history of diatoms that serves to maintain high levels of genetic diversity and to restore large cells to a population. However, complete life cycles of most diatoms are unknown, and observations of sexual reproduction in nature are rare. Culture studies and field observations of Ditylum brightwellii (West ) Grunow in Van Heurck, a centric marine diatom, were used to clarify confusion generated from previous studies of the sexual cycle, to determine the mating system, and to examine natural populations for the incidence of sexual reproduction. Clones of D. brightwellii isolated from Wadsworth Cove, Castine, Maine, were used to observe the morphology of sexual stages. These clones were then studied to determine their mating system and their potential to self-fertilize or outcross. Results from this study show that sexual reproduction in Ditylum brightwellii is homothallic; two naked, spherical eggs and an estimated 64 uniflagellate sperm are released simultaneously from each gametangium into nutrient-replete medium. Oogenesis and spermatogenesis appear to be determined by size; larger clones produced predominately eggs whereas smaller clones produced predominately sperm. Sperm were attracted to the eggs, and size regeneration progressed from a putatively fertilized egg via a true auxospore. Ditylum brightwellii had high gametogenic potential, and self-fertilization was vigorous in some clonal cultures. However, outcrossing was also important in the mating system. Vegetative cell enlargement occurred in older, nutrient-depleted cultures, and produced interme&ate-sized cells. The size structure of natural populations of D. brightwellii in Wadsworth Cove was monitored weekly from 31 August to 7 December, 2004. The size frequency distributions of these populations were mainly unimodal throughout autumn, but appeared to have gradual shifts in size. The smallest cells were found in an early autumn population, whereas, the largest cells, including cells within the size range of initial cells measured in the laboratory (59 a 5 SD pm), appeared in mid-autumn. Spennatogonangia and large, polar cells were found concurrently in Wadsworth Cove in mid-autumn, 2004, suggesting that sexual reproduction was occurring in nature at this time.