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Human Chimpanzee | | Uncapacitated sperm

Human Chimpanzee

Capacitated sperm

Figure 3.5 Left: Red fluorescence values for JC1-stained sperm samples from men and chimpanzees, as measured using flow cytometry. Right: Effects of in vitro induction of capacitation upon JC1 staining (red fluorescence) of human and chimpanzee sperm. *P < .05; **P <.01. Source: After Anderson et al. (2007).

in the sperm midpiece cannot undergo selection, because it is only the female mitochondria which are transmitted to offspring. However, it has to be kept in mind that the genetic programs required to produce the morphological specializations of male gametes, including the volume of the sperm mid-piece, certainly are transmitted to females in the haploid genome contained in the sperm head. The volume of the sperm midpiece can undergo sexual selection via sperm competition, therefore, in order to increase mitochondrial loading and improve sperm motility.

A second point relates to the variability of JC-1 fluorescence when human spermatozoa are stained with this dye. Staining intensity is much more variable among human gametes than is the case for chimpanzees (Anderson et al. 2007). This may relate to the greater morphological variability (pleiomorphism) of human sperm and the higher frequencies of abnormal sperm in human ejaculates. Because sperm pleiomorphism has been implicated in discussions of human sperm competition (Baker and Bellis 1988, 1995), I devote a section to this subject below, in order to place it in true perspective.

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