Abstract
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 studied, 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.
Highlights
It is not exceptional to observe an imbalance in the sex ratio among adult beetles captured in the wild
We studied four of the five species of Cyclocephala endemic to the Lesser Antilles: C. tridentata Fabricius, 1801; C. dominicensis Dutrillaux et al, 2013; C. annamariae Dutrillaux et al, 2013 and C. insulicola Arrow, 1937 and 2 endemic subspecies C. melanocephala rubiginosa Burmeister, 1847 and C. mafaffa grandis Burmeister, 1847
They were captured by light trapping from 1998 to 2011 at the places and dates indicated in Table 1: Some specimens of each species and from most places were kept alive for a few days in wet earth and fed on apple, and brought back to the laboratories of Institut National de la Recherche Agronomique (INRA) (National Institute of Agronomical Research, domaine Duclos, Guadeloupe) or MNHN (Muséum National d’Histoire Naturelle, Paris), where they were used for chromosome studies according to our usual methods (Dutrillaux et al, 2010)
Summary
It is not exceptional to observe an imbalance in the sex ratio (number of males per 100 females) among adult beetles captured in the wild. Males and females may have a different behaviour and visit different biotopes or plants For nocturnal species, such as those of Cyclocephala Latreille, 1829, a frequent bias in sex ratio is a consequence of their sex-dependent attraction to light traps. Parthenogenesis is difficult to demonstrate when it occurs within sexual populations because parthenogenetic females remain attractive and will mate with males. This generally does not affect their reproduction, but exceptionally, a non-reduced (diploid) oocyte may be fertilized, which introduces an additional haploid set of chromosomes from the spermatozoon. The hypothesis that the association between parthenogenesis and insularity is because the latter facilitates colonization by unique females is challenged
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