Title: Sex determination and the restriction of sex-linked pigment patterns to the X and Y chromosomes in populations of a poeciliid fish, Xiphophorus maculatus, from the Belize and Sibun Rivers of British Honduras
Abstract: { Plates I-II; Text-figure 1 ) X. maculatus is polymorphic for sex chromosomes and sex-linked pigment patterns.Females of natural populations may be of the genotype WY, WX, or XX and males XY or YY.Fish from the two rivers were tested for their sex-genotypes, because earlier but limited data had indicated that the X is absent from rivers in British Honduras.Of 8 males and 30 females tested, X chromosomes were only found in one male (XY) and female (XX).One WY female was fertilized by an XY male, before she was collected.Of the 29 females with a W, all but two exhibited one or more sex-linked pigment pattern controlled by at least three loci.The 27 females possessed a total of 41 factors.None was W-linked.In preserved collections from these rivers, the frequency of males with macromelanophore patterns was two to three times that of females.This difference is in good agreement with the hypothesis that in natural populations the W chromosome does not carry pigment factors.This is not true for the X chromosome.Since crossing over beween the W and Y has been observed in the laboratory, it must also occur in natural populations.In the absence of selection, crossing over should tend to equalize the frequency of marked W and Y chromosomes.A selective advantage is postulated for high coloration in males and a disadvantage in females.The significance of the W as a vehicle for strict maternal transmission of characters is discussed.