Morphologically divergent sperm in Odonata: phylogenetic patterns and relations with sperm competition

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In this study, the shape and size of sperm were compared among about 150 species of damselflies and dragonflies (Insecta: Odonata). All sperm observed in damselflies had a usual form as seen in most other insects; however, those observed in dragonflies were atypical, aflagellar or bundled. Changes from normal sperm to aflagellar sperm seemed to occur once on their possible phylogenetic tree. Changes from normal sperm to bundled sperm also occurred once phylogenetically. Genital structure conspicuously differed between the closely related species of damselflies, and this suggested that the genital structure is influenced by some selection pressure in mating behavior. Detailed phylogenetic analyses were conducted for Japanese Mnais and Trigomphus species, based on DNA sequences. The species belonging to each genus were clearly separated by the nuclear DNA, but not by mitochondrial DNA. The incongruent phylogenetic conclusions based on mtDNA and nDNA suggests that introgressive hybridization had occurred between the closely related species in the past. In such a situation, avoidance of hybridization, that is reinforcement of reproductive isolation, may play an important role to advance speciation.