ABSTRACT
At least four species are represented in what used to be called Echinometra mathaei de Blainville, 1825 (Uehara, 1985 & 1988), and the four species have been designated tentatively as A, B, C and D. In each species, two forms of early plutei result from male-female crossing, one with two connective rods that join the right and left body rods, and one with a single connective rod. The average ratios of plutei with single rod and double rods in species A, B, C and D are 1 : 9, 9. 8 : 0.2, 9 : 1 and 8. 5 : 1.5, respectively. Furthermore, the average ratios in nine sets of hybrids among the four species (except hybrids derived from eggs of sp. B) were intermediate between those observed in intra-specific crosses (e.g., the average ratio of single to double rods was 1 : 1 in plutei resulting from the cross A eggs x B sperm). This shows that the egg and sperm nuclei in hybrids participate equally in determining the number of rods. However, based on the ratios in three inter-specific hybrids involving eggs of sp. B, we have shown that the egg cytoplasm has one or more factors that promote the formation of single rod (e.g., the average ratio of single to double rods was 2 : 1 in plutei resulting from the cross B eggs x A sperm). We are now analyzing the pattern of pluteus rod structure in all four species by using parthenogenetically induced plutei.
