The genus Xiphophorus in Mexico and central america.

The genus Xiphophorus is found from northeastern Mexico (Coahuila) for about 2200 Km as far as Honduras. There are 26 species, of which 21 occupy headwaters on the eastern slope of the Sierra Madre Oriental and continuing Cordillera to the southeast. Virtually all the species in the headwaters occupy limited ranges, often in rivers traversing karst country that are separated from lowland streams by underground passages. Only the three forms in the coastal plain are more widely distributed. Nineteen taxa occur within 400 Km of the Mexican Trans Volcanic Belt, suggesting that the genus may have evolved in this region. In many localities two species are sympatric, but natural hybrids are only known from three or four sites. Four monophyletic groups have been identified: the northern platyfish and the northern swordtail groups, north of the Mexican Trans Volcanic Axis, and to the south the helleri and the clemenciae swordtail groups. The status of the three southern platyfish is still not resolved and the phylogenetic relationship of the different groups to each other is still not fully understood.

GENETIC BASIS FOR ALTERNATIVE REPRODUCTIVE TACTICS IN THE PYGMY SWORDTAIL, XIPHOPHORUS NIGRENSIS.

Differences in adult male size and age at sexual maturity in the Río Coy (Mexico) population of Xiphophorus nigrensis (Pisces; Poeciliidae) are controlled by genetic variation at a Y-linked locus. Four genetic size-classes have been identified. The mating behavior of the males of the three largest size-classes consists exclusively of an elaborate courtship display, whereas that of the genetically small males ranges from display to a sneak-chase attempt at copulation. In the presence of large males, small males switch to the sneak-chase behavior. Females prefer the display of large males. In mating-competition experiments (two females with one large male and one small male), the large males are dominant and deny the small males access to females. From 20 such experiments, 601 large-male and 200 small-male progeny were obtained, indicating that the switch to sneak-chase behavior by small males is not particularly effective in overcoming the large-male advantage. By using the largest males of the genetically smallest size class and the smallest males of the genetically next-larger size-class, size was kept constant, whereas genotype was varied. When these males were tested in competition with genetically large males, only the males of the genetically smallest size class showed sneak-chase behavior. These observations suggest that the difference in mating behavior is not an indirect developmental effect of size but, rather, is under direct genetic control.