GENETIC HOMOGENEITY AND SPECIATION IN THE PARTHENOGENETIC LIZARDS CNEMIDOPHORUS VELOX AND C. NEOMEXICANUS: EVIDENCE FROM INTRASPECIFIC HISTOCOMPATIBILITY.

Theoretically, at least, some members of a parthenogenetic species should be genetically identical since every female is capable of generating a clone identical to itself. This assumption of genetic similarity among parthenogenetic individuals formed the basis for several studies using tissue grafts to show absence of an immune reaction and hence evidence of unisexuality in vertebrates. Kallman (1962a,b) first demonstrated clonal structure in a unisexual vertebrate using transplantation tests. He found that the gynogenetic fish Poecilia formosa in the lower Rio Grande is composed of at least six clones, members within each of which possess identical histocompatible genotypes. This interpretation is based on the fact that all tissue grafts are permanently accepted if host and donor belong to the same clone, but are promptly rejected if they belong to different clones. Grafts between mothers and sibs are also successful but not those between "fathers" and sibs since (1) "fathers" belong to a species different from the mother and (2) presumably the eggs are not fertilized. Hence Kallman concluded that P. formosa is unisexual since the offspring do not produce paternal antigens foreign to the female parent. In a similar study using this same species, Darnell et al. (1967) revealed two additional clones in the Rio Guayalejo in northern Mexico further demonstrating maternal inheritance and a unisexual mode of reproduction in P. formosa. Similar evidence was provided by Maslin (1967) to experimentally demonstrate parthenogenesis in lizards. He first showed that a bisexual lizard, Cnemidophorus sexlineatus, rejects grafts from members of its own species (allografts) but retains its own grafts (isografts), thus proving the presence of an immune response to conspecific histoincompatible antigens. Secondly he demonstrated the same response in a parthenogenetic species, C. tesselatus, which rejected grafts from another congeneric species, C. neomexicanus. He then tested for degree of histocompatibility in C. tesselatus between four populations recognized taxonomically as classes A, B, C and E according to Zweifel (1965). All homografts within each clone (class) were permanently accepted as well as those between classes, except for those from classes A and B to C and E. The likely explanation for such results according to Maslin is that all four classes have two genomes in common, but since classes A and B are triploid (Pennock, 1965) they possess one extra and different genome not present in the diploids C and E (Wright and Lowe, 1966). Therefore, C and E can donate to A and B but the reciprocal homografts are rejected. Maslin concluded that individuals of C. tesselatus from one population are histocompatible and that such populations are genetically homogeneous, or nearly so, hence supporting the hypothesis that C. tesselatus is parthenogenetic. Eckardt and Whimster (1971) also demonstrated absence of skin graft rejection in three specimens of the gekkonid lizard Hemidactylus garnoti, supporting Kluge