Reproductive isolation and genetic differentiation of a scleractinian coral Mycedium elephantotus

Sexual reproduction and genetic variation of the scleractinian coral Mycedi um elephantotus at Maoao (MO), Yenliao Bay, northern Taiwan, and at Tiaoshi (TS), Nanwan Bay, southern Taiwan, were studied from 1990 to 1996. M. elep hantotus is a hermaphroditic broadcast spawner with an annual reproductive cycle. Gametogenesis of the MO population began in January, and spawning oc curred in July-August. In the TS populations, there were 2 reproductive gro ups with different timing of gametogenesis and spawning. Gametogenesis of t he first group started in November, and spawning occurred in April or May, while gametogenesis of the second group started in March and spawning occur red in August or September. The temporal reproductive isolation of the 2 gr oups was possibly the byproduct of adaptation. Allozyme electrophoresis was applied to study the genetic variations among regional coral populations a nd the 2 reproductive groups. Significant genetic variations were detected among the MO and TS populations and between the 2 reproductive groups at TS , but no fixed differences were found. The MO population demonstrated large r genetic distances with TS populations (Nei's unbiased D = 0.229 and 0.165 , for the first and second reproductive groups respectively) reflecting the effect of isolation by distance. Significant genetic variation and moderat e genetic distance (D = 0.045) between the 2 reproductive groups at TS indi cate the effect of genetic differentiation due to reproductive isolation in sympatric populations. The lack of morphological differences and the moder ate genetic distance between the 2 reproductive groups suggest recent diver sification within the taxon. The presence of 2 reproductive groups of M. el ephantotus in Nanwan Bay may represent the initial stage of sympatric speci ation in marine environment or a secondary contact of allopatric population s.