MODELS OF RETICULATE EVOLUTION IN THE CORAL GENUS ACROPORA BASED ON CHROMOSOME NUMBERS: PARALLELS WITH PLANTS.

Somatic chromosome number was determined for 22 species of the scleractinian coral genus Acropora, three species of Montipora, and one species of Fungia, using colchicine-treated cells of externally developing embryos. Most had 28 chromosomes, except for six species of Acropora, which had somatic numbers of 24, 30, 30, 42, 48, and 54. Two models that invoke a combination of polyploidy and aneuploidy are presented to account for the observed intrageneric variation in somatic chromosome number. The ability to propagate clones through vegetative fragmentation plus the opportunities for hybridization during multispecies spawning events may have contributed to the development of polyploidy and rapid, sympatric speciation in the uniquely speciose coral genus Acropora. The coral genus Acropora is widespread throughout the tropical Indian, Pacific, and west Atlantic Oceans, where col- onies are typically a dominant component of the shallow reef assemblage (Wallace 1978; Veron 1986). It is the largest of extant coral genera, with more than twice as many species as are found in the next most prolific genera, Montipora and Porites, each with 80 species (Veron 1993; Wallace 1994). The majority (83%) of the 109 extant hermatypic coral genera have fewer than 10 species per genus. Though some authors have suggested that some species of Acropora are recently evolved (Veron 1986) and that Acropora may speciate rapidly relative to other genera (Hodgson 1989), no previous authors have formulated or tested hypotheses that address the unique proliferation of species in this genus. One well-documented mechanism for rapid speciation in plants is hybridization. Veron (1995) has suggested that cor- als are like plants, and indeed they share a number of attri- butes. Asexual propagation through fragmentation or fission is an important reproductive strategy demonstrated both by plants (Stebbins 1950; Grant 1981; Silander 1985) and corals (Hughes and Cancino 1985; Jackson 1985; Jackson and Hughes 1985; Hughes et al. 1992). Similar complexities of sexual reproductive strategies (Veron 1995), the potential her- itability of somatic mutations (Buss 1985; Fautin, in press), and other parallels (Knowlton and Jackson 1993) have in- creasingly stimulated coral biologists to look to terrestrial plants as instructive analogs. Polyploidy is also an important mechanism of speciation in the plant kingdom (Stebbins 1950; Goldblatt 1980; Lewis 1.980b), but is thought to be rare in animals. Might polyploidy be a cytogenetic mecha- nism that has also operated in the diversification of Acropora? Chromosomal studies can provide important evidence for past hybridization events. Two previous studies (Wijsman and Wijsman-Best 1973; Heyward 1985) have attempted to ex- amine scleractinian coral chromosomes, but neither included members of the genus Acropora. In this paper, I present chro- mosome counts for 22 species of Acropora, using mitotic cells of externally developing embryos. Chromosome num- bers were also established for three species of Montipora and one Fungia species. A model that applies established prin- ciples of plant cytogenetics is presented to account for the observed interspecific differences in coral chromosome num- ber.