Male reproduction is challenged by evolution with regard to the formation of „successful” gametes. Amongst these challenges two are most relevant - sperm competition and spermatogonial stem cell (SSC) systems.
While the concept of sperm competition is generally accepted only little is known whether and if by which mechanisms it is reflected in spermatogenesis as such. In principle, three possible mechanisms (or a combination of those) can be suggested: For the sperm, a successful fertilization of the egg occurs when it has won the race. For this purpose, the progressive motility of sperm is crucial, which depends on the number of mitochondria, and the performance of the flagellum, i.e. the length of the sperm. Furthermore, the sheer number of available sperm is a logical determinant which is correlated with the amount of seminiferous epithelium and / or with the rate of sperm production (cycle length).We are interested in the question how sperm competition in testes of primates and birds and have been able to show that there are different adaptations to this challenge in these taxa. While competitive conditions are actually reflected in primates only in relative testes size, birds adjusted the ratio of seminiferous to interstitial tissue and produced sperm of different lengths. Preliminary data also indicate that the duration for the production of mature gametes (spermatogenic cycle length) can vary. The germ line function should also be adjusted for short or long reproductive periods during life. For example, a mouse reproduces only during 1.5 years; monkeys, however, are for fertile many years, increasing the risk of injury or infection. This means that the mission of providing the highest possible number of fertile gametes has to be balanced against maintaining an intact germ line over time. One of the relevant adaptations are different SSC systems, which can be direct (mouse) or progenitor buffered (primates). Our comparative analysis of the organization of the testicular tissue and the endocrine regulation of male reproduction in primates revealed so far unknown findings: On the cellular level and concerning developmental processes, the testicular tissue of New World monkeys corresponds to that of Great Apes (including humans), but differs from Prosimians and Old World monkeys. This could be due to a clonal size and synchrony of SSC derivatives. In contrast, the reproductive endocrine system is very different in New World monkeys from other primates, as luteinizing hormone (LH) is missing and its receptor is mutated, here the Old World monkeys more closely resemble the human situation, via an identical LH / CG - LH receptor system. Thus New World monkeys are a model for research on cellular composition and development of the testis, Old World monkeys, however, should be considered when analyzing hormonal regulation.
Left: Comparative Analysis of primate spermatogenic efficiency proved all monkey species analyzed to exhibit similar rates of sperm production. This indicates sperm competition in monkeys to be reflected only in relative size of testes. Right: New World monkeys have a mutated LHR and developed a reproductive endocrinology dependent on CG, as LH was lost during evolution.