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«Page 1 of Reproduction Advance Publication first posted on 5 November 2013 as Manuscript REP-13-0436 Singular features of fertilization and their ...»

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Since the rate of sperm production broadly relates to testis size, it may be that descent/exteriorization represents something of a trade-off, with a lower level of sperm production offset by a compensatory capacity for sperm storage that maximizes the number immediately available for ejaculationComment.

In contemplating what has determined the characteristics of reproductive systems, the recent literature invokes a process of adaptive co-evolution between the male and female genitalia in groups ranging from Drosophila to waterfowl, with sexual conflict and possibly sperm competiton as other elements. In the case of eutherian mammals, however, the present evidence suggests that adaptive novelties in the form of the sperm head and the mode of fertilization are a fallout rather of the character of the egg – in turn giving rise to others that include morphogenesis of the sperm head features of epididymal sperm maturation, and differentiation of a sperm storage function of the cauda epididymidis that bears on both the need for sperm capacitation and Page 7 of 13 evolution of the scrotum. Such conclusions are supported throughout particularly by a few striking comparative variants on the general pattern.

It has taken many years to appreciate the underlying significance in Eutheria of the special features of the sperm head and of fertilization. As a first element, the cumulus oophorus presents as a dense cell mass essential for induction of the AR in putatively primitive shrews and likely a few other genera, but this has become characterized more often during the mammalian radiations by a matrix that is not essential for fertilization per se but abets in snagging the few spermatozoa at the fertilization site, and/or possibly at ovulstion in egg transport to the tubal ampulla. However, whether the AR begins within the cumulus or on the zona, the interaction thereafter involves several adaptive sperm features established during spermiogenesis and epididymal passage that can be linked to the challenge of penetrating the zona pellucida.

A red flag for understanding the mechanisms involved in zona penetration is the unique configuration of gamete fusion (Fig. 1).

The “equatorial” fusion site appears to have evolved in the face of an unusual physicality of the IAM, which is a trait likely related to shear forces in penetrating the zona. That this penetration involves a major physical component can be inferred from other unusual features of sperm head design - a flat profile and an -S-S- based stiffness - that permit scything-like oscillations of the head within a narowly defined slit, powered by the hyperactivated tail beat. In considering the evidence for zona lysis, such a role for the historical favourite, acrosin, seems unlikely now for the several reasons discussed,, but whether or not penetration in the Eutheria is typically a non-lytic event remains to be resolved. Although the acrosome’s content is discarded before penetrating a zona relatively resistant to proteases, the concept of zona lysis has been hard to abandon as shown by recent claims for various other candidate enzymes, referenced above. However, comparative observations perhaps provide further evidence for the non-lytic version. For example, the rabbit acrosome has a smaller complement of several enzymes (acrosin, hyaluronidase, hexosaminidase and arylsulfatase A) than that of the marsupial opossum (Rodger and Young, 1981), which possesses a thin protease-sensitive zona. This points to an absence of any compensatory selection for acrosomal enzymes with evolutionary hypertrophy of the eutherian zona. Moreover, and illogically for zona lysis, the environment of the Fallopian tube renders the zona more protease-resistant in a variety of species excepting man (Mondejar et al, 2013). Finally, in a few S.E. Asian rodents exemplified by Bandicota indica, a thinner putatively protease-sensitive zona pellucida is matched by a sperm head that is round in profile, has minimal –S-S- crosslinking, and a large rostral acrosome lacking an equatorial segment (Dorman et al., 2013) – all departures from the eutherian norm consistent with a pattern of penetration by zona lysis.

Such adaptations in eutherian spermatozoa are reflected also as new elements in the process of sperm preparation in the male. This is seen during spermiogenesis in the final transition to a uniquely cysteinerich protamine, in the flat profile of the sperm head, in appearance of an insoluble component of the acrosomal matrix, and of the equatorial segment of the acrosome. Further aspects in the epididymis include a subtle reorganization of the acrosomal matrix which may ensure at least an apical marshalling of intra-acrosomal contributors to zona binding (Puigmule et al. 2012), and the possibility for the sperm nucleus to emerge from the reacted acrosome carapace during the first step of zona penetration (Olson et al., 2004). Other novel epididymal events that relate to this interaction include sperm head rigidification based on -S-S- crosslinking within the nucleus and perinuclear matrix, and proteoglycan modifications of the sperm plasmalemma involved in the initiation of sperm/zona binding. In accord with that last point, our efforts to identify glycan-related modifications of the sperm surface were not successful with lectins at least in Suncus murinus - a shrew whose acrosome is shed long before reaching the zona, and in which zona binding is effected by barbs on the perforatorium. In summary, observations in a variety of vertebrates suggest to me that the pattern of sperm maturation in the epididymis may have evolved in two phases – a first simpler sub-therian mode related to issues spermatozoa face in the female tract, complicated then by adaptations to the challenges met in fertilizing the eutherian egg.

–  –  –

Finally the scrotum. As well as the phenomenon of capacitation, a second consequence of the evolution of the cauda sperm storage function appears to be the evolution of the scrotal state. Our experiments in the rat, as an example, demonstrate that temperature-suppression of the cauda epididymidis alone not only removes its ability to prolong sperm viability, but results in 75% fewer spermatozoa in an ejaculation sequence.. Thus, although sperm maturation is not suppressed by it, the storage function is as vulnerable to body temperature as is as spermatogenesis. Why a cooler state should act to enhance the cauda’s potential for sperm storage is not clear but, whatever the explanation, three points are critical to understanding this arrangement.

First, a scrotum has developed only in conjunction with a temperature-regulated sperm storage function: to wit in therian mammals and passerine birds. Second, in the eutherians examined so far, both the scrotal pelage and location of the cauda often favour its cooling over that of the adjacent testis, but never the reverse. Third, although spermatogenesis in the scrotal testis is typically disrupted, it is clear that temperature is not a sancrosanct issue for the therian testis as such in that this can function over a range, each temperature set according to species (Carrick and Setchell, 1977). Given the impression that external migration of the testis helps the cauda of the associated epididymis to “ride” with and often beyond it to a cool location, the notorious sensitivity of the scrotal testis may merely reflect a secondary adaptation to function optimally at the temperature of its location.

On the other hand, the idea that the sperm storage function of the cauda has driven development of the scrotal state is perhaps vitiated by species variation in such as testis size, the extent of its external migration, the prominence of the cauda, and not least in the scrotal pelage. I It is likely that this diversity has been impacted by several factors. In man, for instance, the modesty of the cauda may be exaggerated further by the elevated temperature brought by inguinal clothing. In marine mammals, issues of temperature regulation and streamlining may come into play, and in many species development of the cauda and so sperm delivery has likely been impacted not only by the common practice of polygyny (i.e. the potential need for repetitive ejaculation), but especially by different patterns of sperm utilization within the female tract. However, an explanation for the range in both the size and position of the eutherian testis versus that of the cauda is suggested by the regression analysis of Freeman (1990) in a wide range of species. While it is incorrect to conclude as he does that internal testes produce ‘low quality’ spermatozoa, Freeman’s study provides evidence for a trade-off between the relative size of the testis and so sperm production on the one hand, and the potential for sperm storage on the other His figures indicate that internal testes tend to be relatively larger and produce more spermatozoa, whereas scrotal testes are relatively smaller but with an optimal cauda sperm store compensating for this deficit. In other words, low-temperature-enhanced epididymal storage has reduced the pressure to produce spermatozoa without compromising the number ejaculated. This paradigm is epitomized by the situation in the rat compared to that in the similarlysized Japanese quail (Clulow and Jones, 1992), both of which can ejaculate some 300 million spermatozoa within a limited period. The quail achieves this by producing a greater number from a larger testis with rapid epididymal transport, whereas the smaller scrotal testis of the rat produces fewer spermatozoa with slower throughput, but equal numbers in an ejaculate series thanks to the cauda’s sperm store.


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Maturation, Motility, Surface Properties and Comparative Aspects pp 7 - 21. Eds DW Fawcett and JM Bedford.

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