Sex ratio and chromosomes studies were performed on six of the seven species of genus Cyclocephala present in the Lesser Antilles. Most sex ratios based on light trapping are skewed, with a frequent, but not exclusive, strong excess of females. These unbalances vary from species to species and island to island for the same species. Comparing old (seventies) and recent captures, the imbalances seem to increase with time. All of the 72 karyotyped males were diploid: 20,XY. Of the 15 females

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... d, one (C. dominicensis) was triploid: 30,XXX, a condition that only occurs in parthenogenetic species of beetles. We conclude that parthenogenesis is progressively developing within different sexual populations of Cyclocephala in the Lesser Antilles, which accounts for the excess of females recorded there. We propose that preexisting recessive mutations are the cause and that island colonization, by preventing panmictic reproduction, favours the expression of these recessive mutations. This would account for the accelerated occurrence of new characters (speciation /sub-speciation process), including unusual modes of reproduction such as parthenogenesis, and that several genetically related populations express these characters independently on different islands.