Structure, function, and evolutionary significance of the reproductive system in males ofHebrus ruficepsandH. pusillus(heteroptera, gerromorpha, hebridae)

AbstractIn Holland, bugs of the speciesHebrus pusillusandH. ruficepshave one generation per year and overwinter as unmated adults. Males have two testes with two follicles + vasa efferentia each, paired vasa deferentia and seminal vesicles, an ejaculatory duct, and a protrusible phallus comprising an articulatory apparatus, phallotheca, endosoma, and paired claspers. The skeletomusculature of this system is described (it has 12 paired and four unpaired muscles) and its functions in generating and transferring sperm (summarized in Figs. 70–75) are reconstructed from study of living bugs, dissections, whole mounts, and serial sections.Males of both species produce sperm >2 mm long from stem spermatogonia in the germarium of each follicle. Initial definitive spermatogonia divide synchronously three times to form clones of eight, interconnected, primary spermatocytes. These enlarge up to 43‐fold in males ofH. pusillusand 78‐fold in those ofH. ruficeps, undergo meiosis, and, after adult emergence, complete their differentiation into bundles of 32 sperm which coil transversely about the periphery of each follicle at its base. These begin to enter the vasa efferentia in mid August, rupture, and release their sperm into the seminal vesicles where they are stored overwinter. Most spermatocyte and spermatid cysts remaining in the testes degenerate in fall, leaving only stem spermatogonia and a few early spermatocysts in the germaria.Males ofH. pusillusbegin to mate the first warm days of spring but only the most persistent succeed. A male jumps on the back of a female, induces her to lower her ovipositor, and, within 12 min (@ 18–24°C), introduces the endosoma of his phallus up its shaft and fills his seminal duct with sperm. The female draws this into her gynatrial sac at the end of copulation and transfers it into her spermatheca in about 30 min, the sperm reversing themselves within it so that their heads face towards its mouth. The male may stay on her back for up to 2 hours and may copulate again up to three times before leaving to mate with other females.Males ofH. pusillusmay be sexually active for months after overwintering, because spermatogonia in their germaria reactivate in spring to produce additional sperm. Those ofH. ruficepsdo not and males mate successfully only until their supply of overwintered sperm is exhausted.The chromosome complement ofH. pusillusmales is 2N = 22 + XY. The X and Y chromosomes are of unequal length, form a pseudo pair at metaphase I, and segregate to opposite poles at anaphase I—the first instance of pre‐reductional segregation of sex chromosomes to be recorded in the Gerromorpha. The chromosome complement ofH. ruficepsmay be 2N = 24 + XO but the nature of two chromosomes was not resolved. The single X segregates to half the spermatids at anaphase II.