Frontiers in Reproductive Science; Reproductive Biology, Physiology and Biochemistry of Male Bats

Spermatogenesis is a highly regulated and synchronized process of cellular division and differentiation whereby spermatogonia proliferate, meiotically divide, and gradually differentiate into highly specialized haploid cells, known as spermatozoa. Its main events are generally similar in most mammals, and it may be divided into three phases: the proliferation phase, the meiotic phase, and spermiogenesis. The proliferation phase corresponds to the process of stem cell renewal, where spermatogonia mitotically divide to both replenish the stem cell pool and to originate cells that are subject to further differentiation. The meiotic phase is the process in which spermatocytes undergo meiotic divisions that give rise to haploid spermatids. Moreover, spermiogenesis is the process in which a round spermatid differentiates into a mature spermatozoon that is capable of motility and fertilization. The process of spermatogonial differentiation in bats is relatively similar to that found in primates, with three main types of spermatogonia: Ad, Ap, and B. Meiotic divisions proceed similarly to those of most mammals, and spermiogenesis can be divided into 9 to 16 steps. Despite these similarities, some species-specific variations are observed. Bats present three different processes for the formation of the acrosome, and the ultrastructure of spermatozoa has been found to have unique characteristics, including many wavy acrosomal projections on the acrosome; surface of the family Molossidae, an extraordinarily large head with accordion-like folds of the Noctilionidae, differences in the degree of development of the outer dense fibers, and the presence/absence of a perforatorium.

The reproductive process requires the coordinated action of a large variety of peptides and steroid hormones, each one playing an important role in the normal functioning of the reproductive organs. In male bats, this control is linked to the correct functioning of the hypothalamic-pituitary-testicular axis, combined to the paracrine communication between Sertoli, Leydig and germ cells. The hypothalamus produces and secretes gonadotropin-releasing hormone (GnRH), which acts on the pituitary gland stimulating the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones act directly on the testicular somatic cells in order to stimulate and regulate their function and to control the spermatogenesis process. Testicular steroids (androgens and estrogens) in turn stimulate the seminiferous tubules and all secondary sex structures (epididymis and reproductive accessory glands, mainly the prostate), as well as provide feedback to the hypothalamus and pituitary. The hormonal regulation of bats differs interspecifically, with some species presenting the typical mammalian pattern; others showing unique variations, possibly imposed by the hibernation period; until species that have developed different adaptations, but without the imposition of a period of hibernation. One of these adaptations is the testicular regression, which is a deactivation process of the testicular tissue that is triggered by changes in the pattern of hormonal regulation. Thus, in this chapter, we seek to discuss different patterns of hormonal regulation of bats and some aspects of testicular regression.

Leptonycteris yerbabuenae is a bat with a distribution range from southern USA to El Salvador. A distinctive feature of the species is the seasonal accumulation of subcutaneous fat. Since this specie does not hibernate, it has been speculated that this energy reserve is used for migration and reproduction or when food availability decreases. As the role of fat in populations of southern Mexico is unknown, we carried out this study to determine the fat accumulation pattern and the role it plays during an annual cycle in a resident colony of this taxon. Results show that males follow a seasonal pattern, which begins with fat accumulation in February and peaks in May, right when the mating period begins. After this period, the amount of fat steadily decreases and disappears during November-January. We also found that the body and testicular mass are positively related with the amount of accumulated fat and with the development of an interscapular dorsal patch. Data suggest that stored fat is used for reproductive purposes and as an energy reserve to stay longer in the breeding colony, and that the interscapular dorsal patch may be taken as the evidence of reproductive success.

In the species of bats that present a asynchronous reproductive pattern (Corynorhinus mexicanus), spermatogenesis is in recess during autumn and winter, when androgen levels are low, but the secondary sexual functions are maintained and mating takes place. The proliferation of spermatogonia and the spontaneous degeneration of spermatocytes often coincide during normal testicular functioning in mammals, and this is where apoptosis plays a central role in controlling the number of testicular germ cells. The spermatogonia are the cells that frequently suffer apoptosis in the testicles, but this process can also affect spermatocytes and spermatids. This emphasizes the need to improve our understanding of the participation of apoptosis and, therefore, its modulation during testicular development and training of new blood vessels. The strict regulation of angiogenesis and its adequate functioning are very important for organisms, since both the excessive formation and the insufficient development of blood vessels can cause severe diseases. Studies of the formation and involution of testicular blood vessels in seasonal reproductive bat species may thus shed new light on understanding the control and development of angiogenesis in mammals. Using the C. mexicanus bat as a study model of the seasonal activity of the testicle, we determine for the first time the involvement of the apoptosis in developing testicular vasculature in a seasonal chiropteran with drastic morphological changes of the testes, although more comparative studies are needed, mainly one between the different stages of testicular cycle which can provide information of the existence of angiogenesis associated with the testicular cycle in these species.

After spermatogenesis within the seminiferous tubules of the male gonads, the sperm cells continue their journey through the male reproductive tract, until ejaculation, however, described for the generality of mammals studied, that transit through the epididymis is essential for sperm to acquire the potential to carry out hyperactivation and acrosome reaction within the female genital tract and, in turn, fertilize the egg. The sperm is a transcriptionally silenced cell, so it depends almost entirely on proteins that are acquired or modified during their tour of the epididymal duct. Recently, studies have been conducted using the bat (Corynorhinum mexicanus) as a study model, which has allowed to explore some important details of the process of epididymal sperm maturation, such as the fact that in this species, the epididymal maturation ends in the caudal region, and not as had been established so far in other groups of mammals, another aspect to highlight is the participation of the reactive oxygen species and the different enzymes involved, which have been reported to play a crucial role in the signaling pathways within the maturation of sperm. Also, studies have been conducted on post-translational changes that present the proteins that make sperm and its various organelles, fundamentals for the survival of the cell and its recognition with the ovum.

Prolonged sperm-storage is an adaptation to the reproductive cycle that is most highly developed in bat. Twenty four bat species have been identified so far to store spermatozoa for the period of approx. 16 days up to ~225 days and still retain fertilizing capacity. The aim of this review is to evaluate the mechanism and molecules necessary for sperm to become efficiently stored in female genital tract. Spermatozoa are arranged with their head towards the reproductive tract epithelium are characterized in all sperm storing bat species so far investigated, suggesting that such intimate relationships are an integral part of the mechanism of prolonged storage of sperm. Recent study suggests that the ultimate controls of the mechanism underlying spermstorage are hormonal. It has been demonstrated that sperm stored in female bat contains high circulating level of androgen during the period of sperm storage. It is suggested that androgen creates a unique microenvironment that facilitates prolonged sperm storage. The identification of the specific molecules responsible for prolonged spermstorage may suggest a mechanism to keep the sperm viable for prolonged period for use in assisted reproductive technique.

Some of the most striking behaviours and characters in nature are the result of sexual selection, including sexual dimorphism in size, pigmentation, ornamentation and weaponry. Our understanding of sexual selection is largely based on birds and insects research and, of the mammals, primates and ungulates. Bats have received comparatively little attention. Indeed, bats may not appear to be the most obvious model for studying sexual selection because males lack the exaggerated weaponry seen in some mammals, and typically lack visually striking ornamental traits. However, bats are well placed for studying two key aspects of sexual selection, sperm competition and cryptic female choice. Here we introduce these processes, before explaining why they may be particularly important in bats. We then review research on sperm competition and cryptic female choice in bats and highlight the tremendous opportunities to improve our understanding further by studying the Chiroptera.