Abstract
ABSTRACTWe assess karyotypic variation in the heteropteran infraorder Gerromorpha. Counts of autosomes, m-chromosomes and sex chromosomes are obtained from literature reports for 51 species, including 35 in the family Gerridae. Placing these data on recently derived phylogenies reveals that XX/X0 sex determination is ancestral to the clade containing the Gerridae and Veliidae, but that XX/XY sex determination has been secondarily derived several times. Our results confirm the absence of m-chromosomes in the Gerromorpha, but contrary to previous reviews, we find intermediate levels of variation in chromosome counts and sex-determining systems when compared to other Heteropteran clades. A new karyological analysis reveals that the X chromosome of Aquarius remigis Say, 1832 is the largest chromosome, which contrasts with A. najas De Geer, 1773 in which it the X chromosome is the smallest. This karyotypic difference supports molecular evidence that the A. remigis group of species is not closely related to other species in the genus Aquarius Schellenburg, 1800.
Highlights
We report the chromosome number and mechanism of sex determination for 51 species within the heteropteran infraorder Gerromorpha
The chromosome complement of 2n = 20A + X0 that we observed for male Aquarius remigis is typical of species in the family Gerridae with respect to both the number of autosomes and the sex-determination system
Because the chromosome counts and the sex-determination system are so conserved in the Gerrinae, the A. remigis karyotype is not informative with respect to its phylogenetic position as a sister clade of Gerris (Figure 3; Damgaard et al 2014)
Summary
We report the chromosome number and mechanism of sex determination for 51 species within the heteropteran infraorder Gerromorpha. This report updates previous reviews of karyotypic variation in the Gerromorpha which included only 21 (Ueshima 1979; Andersen 1982) or 34 (Papeschi and Bressa 2006) species. (but not always) minute compared to autosomes and are distinguished by meiotic behavior that differs from both autosomes and sex-chromosomes. Among other peculiarities, they are achiasmatic, as are sex-chromosomes in the Heteroptera, but unlike sex-chromosomes, they undergo reductional separation at the first division and equational separation at the second (for more detailed descriptions of the meiotic behaviors of autosomes, sex-chromosomes and m-chromosomes, see Ueshima (1979)
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