Reference: Biol. Bull. 2(14: 10-20. (February 2003) 2003 Marine Biological Laboratory Molecular Genetic Evidence that Dinoflagellates Belonging to the Genus Symbiodinium Freudenthal Are Haploid SCOTT R. SANTOS* AND MARY ALICE COFFROTH4 Department of Biological Science, State University of New York at Buffalo. Buffalo. New York 14260-1300 Abstract. Microscopic and cytological evidence suggest that many dinoflagellates possess a haploid nuclear phase. However, the ploidy of a number of dinoflagellates remains unknown, and molecular genetic support for haploidy in this group has been lacking. To elucidate the ploidy of symbiotic dinoflagellates belonging to the genus Symbioiliniinii, we used five polymorphic microsatellites to examine popula-tions harbored by the Caribbean gorgonians Plexauni kuiui and Pseudopterogorgia elisubetluie; we also studied a series of S\mbioiUnium cultures. In 690 out of 728 Symbiodinium samples in hospite (95% of the cases) and in all 45 Sym-biodinium cultures, only a single allele was recovered per locus. Statistical testing of the Symbiodinium populations harbored by P. elisabethae revealed that the observed ge-notype frequencies deviate significantly from those ex-pected under Hardy-Weinberg equilibrium. Taken together, our results confirm that, in the vegetative life stage, mem-bers of Symbhidinium, both cultured and in hospite. are haploid. Furthermore, based on the phylogenetics of the dinoflagellates, haploidy in vegetative cells appears to be an ancestral trait that extends to all 2000 extant species of these important unicellular protists. Introduction The ploidy of an organism can significantly affect ge-nome evolution. For example, diploids carry twice as much DNA as haploids and may be expected to accumulate new beneficial mutations at a higher rate (Paquin and Adams. Received 18 July 2002; accepted 2S November 2002. * Present Address: Department of Biochemistry and Molecular Biophys-ics. University of Arizona. Tucson, Arizona 85721. :fTo whom correspondence should he addressed. E-mail: coffroth@ hutlalo.edu 1983). The genome of diploids may also evolve rapidly because they carry more than a single copy of an allele. These "extra" alleles, over time, may evolve new functions while "old" alleles continue to perform their original func-tions (Lewis and Wolpert. 1979). Haploids. on the other hand, tend to have deleterious mutations purged more rap-idly from the population since they are not masked (Hughes and Otto. 1999). Furthermore, knowledge of ploidy is es-sential to the interpretation and understanding of population genetic data. Given the importance of ploidy to genome evolution and population genetics, it is surprising that ques-tions pertaining to it remain for a number of organisms. Dinoflagellates are a diverse and ecologically important group of unicellular protists. For example, some species are major photosynthetic or heterotrophic components of the plankton, and others are considered to be the causative agents of fish kills. Microscopic and cytological evidence from the species examined to date suggests that dinoflagel-lates (with the exception of Noctiluca spp.) possess a veg-etative haploid nuclear phase (Pfiester and Anderson. 1987; Coats, 2002). However, ploidy has not been explicitly de-termined for a number of dinoflagellates, including the important genus Symbiodinium Freudenthal (Taylor, 1974). Members of Svmbiodinium. commonly referred to as zoo-xanthellae, are intra-or intercellular symbionts of marine invertebrates, including foraminiferans, sponges, cnidar-ians, and molluscs (Glynn, 1996). Blank (1987) recon-structed the nucleus of Symbiodinium kawagutii and found that the chromosomes of this dinoflagellate could not be paired either by size, appearance, or distribution. This cy-tological result led to the speculation that the vegetative (coccoid) cells of Symbiodinium may be haploid (Blank, 1987). To date, the molecular genetic data necessary to establish the ploidy of these symbiotic dinoflagellates, or 10
BioStor
