Daily production of spermatophores, sperm number and spermatophore size in two eriophyoid mite species

Under dissociated sperm transfer, (non-pairing) males deposit spermatophores on a substrate, while females seek spermatophores and pick up sperm on their own. Spermatophore expenditures of non-pairing males should be high, due to the increased uncertainty of sperm uptake by a female. In this study I examined spermatophore expenditures in two eriophyoid species that differed in the degree of dissociation between sexes: (1) Aculus fockeui (Nalepa and Trouessart) males rarely visit quiescent female nymphs (QFNs), and mostly deposit spermatophores all over the leaves, whereas (2) Aculops allotrichus (Nalepa) males guard QFNs for many hours and deposit several spermatophores beside them. Males of both species were collected from the field and tested in solitude. Aculus fockeui males deposited on average 19.1 spermatophores per day, whereas A. allotrichus deposited only 3.6 spermatophores per day, and had a very large coefficient of variation. Males and spermatophores of A. allotrichus were significantly smaller and contained less sperm than those of A. fockeui. In both eriophyoids, spermatophore size was fitted to the size of female genitalia and the height of females. The ratio between the diameter of spermatophore head and the width of a female genital coverflap was 0.6, whereas the ratio between the female leg and the length of spermatophore stalk was 0.5. Several factors could be responsible for the discrepancy in spermatophore expenditures between species. Among other factors, the effects of male size, male reproductive strategy and female genitalia size on spermatophore output and size of spermatophores are discussed.

The evolution of male genitalia: patterns of genetic variation and covariation in the genital sclerites of the dung beetle Onthophagus taurus

Three main hypotheses, have been invoked to explain divergent genital evolution, the lock and key, pleiotropy, and sexual selection hypotheses, each of which make different predictions about how genital traits are inherited. Here we used a half-sib breeding design to examine the patterns of genetic variation and covariation between male genital sclerites, and their covariance with general body morphology in the dung beetle Onthophagus taurus. We found CV(A)'s and CV(P)'s were similar for both genital and general morphological traits and that CV(R)'s were large for both trait types. We found that male genital sclerites were negatively genetically correlated with general morphological traits. Variation in male genital morphology has direct implications for a male's fertilization success and the resulting sexual selection acting on male genitalia is predicted to maintain high levels of additive genetic variance. Contrary to this prediction, we found that individual genital sclerites all had low levels of additive genetic variance and large maternal and environmental sources of variation. Our data suggest that the genital sclerites in O. taurus are not inherited independently but as a genetically integrated unit. More importantly, the way the different sclerites function to influence male fertilization success reflects this genetic integration. Even though levels of V(A) in individual genital sclerites may be low, there may still be sufficient V(A) in multivariate trait space for selection to generate evolutionary change in the overall morphology of male genitalia.

Genital morphology and fertilization success in the dung beetle Onthophagus taurus: an example of sexually selected male genitalia

In animals with internal fertilization and promiscuous mating, male genitalia show rapid and divergent evolution. Three hypotheses have been suggested to explain the evolutionary processes responsible for genital evolution: the lock-and-key hypothesis, the pleiotropy hypothesis and the sexual-selection hypothesis. Here, we determine whether variation in male genital morphology influences fertilization success in the dung beetle Onthophagus taurus, as predicted by the sexual-selection hypothesis. Variation in four out of five genital sclerites of the endophallus influenced a male's fertilization success, supporting the general hypothesis that male genitalia can evolve under sexual selection. Furthermore, different genital sclerites were found to enhance first versus second male paternity, indicating that different sclerites serve offensive and defensive roles. Genital-trait variability was comparable to that in other species but was less variable than a non-genital sexually selected trait (head horns). We suggest that directional selection for genital elaboration may be countered by natural selection, which should favour genitalia of a size and shape necessary for efficient coupling and sperm transfer.

Indirect sperm transfer in arthropods: behavioral and evolutionary trends

Arachnids, myriapods, and wingless hexapods exhibit a fascinating diversity of sperm transfer behaviors. Modes of sperm transfer can be categorized by the degree of contact between male and female during transfer, with direct transfer (copulation) involving the greatest contact, paired-indirect transfer an intermediate degree, and dissociated transfer the least. Internal fertilization, spermatophores, and copulation are sometimes assumed to have evolved after invasion of land, but all have evolved many times in the marine habitat. Behaviors associated with indirect sperm transfer include those having close parallels with direct transfer (provision of nuptial gifts) as well as unique phenomena (spermatophore trampling by rival males). The morphology and physiology of indirectly transferred spermatophores have been shaped by environmental factors (e.g. humidity) as well as biological ones (e.g. clutch size of females), and they may provide useful phylogenetic characters. Unanswered questions about indirect transfer include the following: Are females of dissociated species able to choose their partners? What determines size and number of spermatophores? Do speciation rates differ between taxa with direct versus indirect transfer?

Formation, transfer, and assimilation of the spermatophore of the hermaphroditic flatworm Dugesia gonocephala (Tricladida, Paludicola)

The spermatophore of the simultaneously hermaphroditic planarian Dugesia gonocephala consists of a large stalked bladder that gradually forms inside the ejaculatory duct of the penis between successive copulations. In solitary individuals, spermatophore formation begins with the development in the penis of a tube that is sealed at the distal end and may protrude slightly from the penis tip. In courting individuals, this protruding part is widened into a bladder that further increases in size when penises are mutually inserted. Spermatozoa are pumped into the bladder after the first few minutes of copulation and the resultant spermatophores are exchanged 3–4½ h after the start of copulation. Reciprocal transfer is slightly asynchronous. After the spermatophore is transferred into the bursa copulatrix, the stalk of the spermatophore is usually in or near the entrance to the bursal canal. Sperm start to leave the spermatophore immediately after the partners separate and continue to do so for 3–4 days. During this period, the bladder of the spermatophore shrinks but remains intact. Following spermatophore donation, the penis may hold a new spermatophore after 1–2 days. Individuals that engage in a new copulation rarely do so because they are allosperm depleted. In fact, the bursa may still hold a spermatophore from a previous copulation. We discuss the adaptive significance of spermatophore formation, transfer, and assimilation.