Although the genomes of Chlamydomonas and Volvox are similar in most ways, there is one glaring exception that provided the Salk researchers with an entrée into the origin of male and female sexes-the so-called mating locus that functions in much the same way as human X and Y chromosomes to determine gender.When Umen and his colleagues examined the mating locus genes in Chlamydomonas and Volvox they found that they shared some of the same genes, as you would expect from closely related species. However, Volvox also now possessed a surprising variety of new genes that were added to its expanded mating locus, and expression of many of these genes had come under the control of the male or female differentiation programs.
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