Variation in chromosome numbers and the sex determination system in the Gerromorpha with special reference to the family Gerridae (Hemiptera)

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 oth...