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The fishes of the genera Platypoecilus and Xiphophorus constitute the tribe Xiphophorini. The six species of xiphophorin fishes may be regarded as geographical species as they occupy distinctive drainage areas in Mexico. The chromosome numbers in males of all the species of the tribe Xiphophorini were determined at the first maturation division in males. In Platypoecilus couchianus, P. xiphidium and P. maculatus, and in Xiphophorus hellerii and X. montezumae the haploid number of chromosomes is twenty-four. In P. variatus the haploid number is twenty-five, the additional univalent chromosome appearing smaller than the bivalents. The chromosomes of F1 hybrids between P. variatus and P. maculatus are described. The extra chromosome may, together with another variatus chromosome, associate with a maculatus chromosome to give a trivalent, or it may remain unpaired as a univalent. The chromosomes of these species are small and the complements they form are quite similar in appearance. Therefore no cytological characters were found that distinguish the various species of the tribe Xiphophorini, except P. variatus. The similarity of the xiphophorin chromosomes partly explains the ease with which specific and generic crosses are made in this group, and also, the high degree of fertility that is shown by the first generation hybrids.
The American Naturalist © 1934 The University of Chicago Press