Sexual selection and sperm midpiece volume

The sperm midpiece contains a tightly packed, helical array of mitochondria, which surround the central strut, or axoneme, of the sperm. The mitochondria differ in size and number between species. As examples, Figure 3.3 shows electron micrographs of the sperm midpieces of the monkey and the squirrel chimpanzee in longitudinal section. The squirrel chimpanzee sperm midpiece contains fifty-three mitochondrial 'gyres' (i.e. spiral folds around the central axoneme) by comparison with only twenty-two in the chimpanzee. However, in the chimpanzee the mitochondria are much larger than those of the squirrel monkey. These mitochondria are the sites of oxidative phosphorylation and production of energy. They are not the only potential source of energy for sperm motility, as glycolysis also plays an important role in this process (Miki et al. 2004). However, it follows that differences in mitochondrial loading in sperm of different species may be significant in functional and evolutionary terms. Might it be possible, for example, that larger volumes of mitochondria occur in the sperm mid-pieces of mammals which engage in sperm competition? More vigorous or more sustained motility of sperm with larger midpieces might be significant in competitive contexts. Unfortunately, attempts to implicate linear measurements of sperm midpieces

Figure 3.3 Electron micrographs of longitudinal sections through the sperm midpiece of (A) the squirrel monkey (Saimiri sciureus), and (B) the chimpanzee (Pan troglodytes). Note the pronounced differences in the numbers and sizes of the mitochondria. Source: After Bedford (1974).

Figure 3.3 Electron micrographs of longitudinal sections through the sperm midpiece of (A) the squirrel monkey (Saimiri sciureus), and (B) the chimpanzee (Pan troglodytes). Note the pronounced differences in the numbers and sizes of the mitochondria. Source: After Bedford (1974).

are of little value in addressing these questions (Malo et al. 2006). As indicated by the squirrel chimpanzee-monkey example discussed in Figure 3.3, volumetric measurements provide a more useful guide to the likely degree of mitochondrial loading. This method is not perfect, of course, because the midpiece consists of a dense sheath, enclosing the central axoneme as well as the mitochondria. However, if the midpiece is treated as a cylindrical structure, then its overall volume may be estimated from measurements of its width and length. Comparative measurements of the sperm of primates and of many other mammals confirm that larger midpieces occur in species where females mate with multiple partners, and where sperm competition is also associated with larger testes sizes (Figure 3.4). The positive correlation between sperm midpiece sizes and relative testes sizes is especially interesting. Thus, it appears that sexual selection, via sperm competition, has resulted not only in the ability to produce larger numbers of sperm, but also sperm which have larger midpieces and a greater mito-chondrial loading for energetic purposes.

The findings summarized in Figure 3.4 derive from measurements of the sperm of 123 species of mammals (Anderson and Dixson 2002; Anderson et al. 2005) on substantial numbers of gametes (100 sperm for each male) and using a single, consistent methodology. Previous failure to establish this relationship (Gage and Freckleton 2003) may have resulted from the use of sperm measurements derived principally from the published literature. Unfortunately, this produced a dataset which contained inconsistent measurements of sperm dimensions. The same problem has hampered studies of human relative testes sizes, as was discussed in the previous chapter with regard to inaccuracies arising from the use of orchidometers (see Table 2.2). Nor do the findings on mammals presented in Figure 3.4 necessarily apply to all vertebrate groups. Thus, sperm midpiece volumes and relative testes sizes are not correlated in passerine birds (Immler and Birkhead 2007). The spermatozoa of birds are, of course, morphologically very different to those of mammals (Jamieson 2007).

Human sperm have relatively small midpieces, by comparison with those of many other primates, and in relation to testes size (Figure 3.4, upper part).

Sperm midpiece volumes of human beings, the apes, and other anthropoids are included in Table 3.1. Human sperm midpieces are smaller than those of all the other hominoids, including the gorilla and orangutan. The human sperm midpiece contains fifteen mitochondria (Bedford 1974; Bedford and Hoskins 1990) and these are substantially smaller, in overall volume, than the mitochondria of the chimpanzee. Thus the data on human sperm morphometry indicate that sexual selection, via sperm competition, is unlikely to have played a significant role in human evolution. This finding should give pause for thought to those authors who maintain that human testes size reflects effects of sexual selection because it is larger than that of the gorilla, or slightly larger than that of the orangutan.

Recent research on sperm energetics using live gametes from chimpanzees and human beings has provided additional support for hypotheses concerning effects of sexual selection upon midpiece sizes and mitochondrial loading. Living sperm from four men and five chimpanzees were treated with JC-1, a fluorescent dye which stains metaboli-cally active mitochondria red/orange. Intensity of staining provides a measure of mitochondrial membrane potential and has been shown to correlate with forward motility in human sperm (Mar-chetti et al. 2004). Comparisons of chimpanzee and human sperm show that fluorescence intensities in chimpanzees are markedly higher, due to bright red staining of mitochondria in the midpieces of their sperm (Anderson et al. 2007). Results are shown in Figure 3.5. These differences persist in sperm which have undergone (in vitro) capacitation, a process which normally occurs within the female reproductive tract and which prepares spermatozoa physiologically to fertilize ova. Whereas mito-chondrial staining by JC-1 tended to decrease after capacitation in human sperm, this was not the case in chimpanzees (Figure 3.5). Capacitated sperm are capable of a distinctive kind of vigorous motil-ity, often called hyperactivated motility, and which normally occurs in the oviduct prior to fertilization (Yanagimachi 1994). Whether the higher mito-chondrial loading of chimpanzee sperm improves their motility in lower portions of the female tract, or their capacity for hyperactivated motility in the oviduct, requires further study. However, the JC-1

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