I read with great interest the article by Charles E. Boklage entitled ‘The epigenetic environment: secondary sex ratio depends on differential survival in embryogenesis’ (Boklage, 2005) and I consider it interesting but I am concerned about some of his conclusions. The author concludes that the excess of human males at birth does not arise in gametogenesis or fertilization, and must therefore be generated in embryogenesis by a preferential loss of female embryos. In my opinion, the excess of males may be produced during fertilization, and differential survival in early embryogenesis may occur in the opposite way. These are my arguments:
I agree with the author that the theoretical coefficient between X and Y chromosomes in the ejaculates is 1:1 and that it has been experimentally demonstrated in mammals that during meiosis equal numbers of X and Y sperm are produced. However, there are differences between X- and Y-bearing sperm that have been used to separate these populations. For example, the X chromosome is larger than the Y chromosome and therefore contains more DNA. It may be expected that differences in DNA mass between X and Y chromosome-bearing sperm would influence swimming speed. Among those methods devised to separate X- and Y-bearing sperm by apparent physical characteristics, the albumin gradient method described by Ericsson et al. (1973) is based on the faster swimming speed of the smaller Y-bearing sperm, which enables those sperm to reach the bottom of the gradient before the X-bearing sperm. This method has been offered to clinicians for many years as an effective method for sex preselection. Also, several reports indicate a sex-related differential characteristic in human sperm allowing separation of X- and Y-bearing sperm using swim-up (Han et al., 1993) and producing a high percentage of male births after insemination with sperm isolated by a modified swim-up procedure (Check et al., 1994).
We have previously reported that there is an association between maturation stage of bovine oocytes at the time of IVF and sex ratio of in vitro-derived embryos (Gutierrez-Adan et al., 1999). We hypothesized that the differential ability of X- or Y-bearing sperm to fertilize oocytes is due to differences in the physiological activity (motility/viability or capacitation/acrosome reaction) of X- or Y-bearing sperm before fertilization. Even though during meiosis in mammals equal numbers of X- and Y-bearing sperm are created, this might not reflect the proportion of the sperm population that reaches oocytes in the oviduct. There is also some evidence confirming the existence of differences between X- and Y-bearing sperm; it has been reported that in a simple salt solution, Y-bearing bull sperm do not swim faster than X-bearing sperm but may be distinguished from X-bearing sperm on computer-assisted sperm analysis (CASA) on the basis of linearity (LIN) and straightness of path (STR) (Penfold et al., 1998); also we have reported that using a double swim-up sperm preparation method we can obtain differences in the percentages Y-chromosome DNA-bearing sperm in some of the sperm fractions, suggesting that there are intrinsic differences in capacitation of X- and Y-bearing sperm that might be used to produce embryos of the desired sex with IVF (Madrid-Bury et al., 2003).
The primary sex ratio (male:female ratio at the time of fertilization) in humans differs remarkably from the theoretically expected ratio of 1:1, and may be as high as 170 males to 100 females (Pergament et al., 2002). Because the secondary sex ratio (or ratio at birth) is ∼106:100 in the majority of developed countries, it is clear that preimplantation and prenatal mortality affects sexes differently. It has been suggested that under some stress conditions male embryos are more vulnerable than females. Maternal conditions and environmental and stressful circumstances affect sex ratio in mammals (James, 1998). External maternal stress around the time of conception is associated with a reduction in the male:female sex ratio, suggesting that the male embryo is more vulnerable than the female (Hansen et al., 1999). Sex ratio declines as a consequence of environmental pollution, destructive earthquakes, smoking parents, aged mothers, stress caused by ovulation induction procedures, war situations, etc. (James, 1998). In our opinion, a greater attrition of males is exerted in all those situations in which reproductive conditions are suboptimal. Food restriction in animals also produces female-biased litters. In rodents it has been seen that stresses other than food restriction can also reduce the proportion of males, and that pregnant rodents respond to stress showing a selective prenatal vulnerability of male fetuses as is observed in many mammals, including humans, suggesting that male embryos and fetuses are particularly susceptible to loss (James, 1998; Pergament et al., 2002).
The fact that stress favours daughters is consistent with the sex allocation hypothesis of Trivers and Willard (1973); their sex ratio theory predicts that natural selection in mammals should favour an excess of male offspring only when mothers are in good survival conditions, whereas endangered mothers would benefit by producing daughters; thus it is conceivable that parents could adjust the sex of their offspring in response to environmental conditions. If the mechanism that controls the sex ratio were only functional after fertilization, in most monotocous mammals which shed one ovum per ovulation, implantation failure would necessarily result in infertile ovarian cycles. Our hypothesis is that under normal or good conditions the sex ratio variation is determined pre-conceptionally, favouring the sex of greater variance (in terms of early mortality and reproductive success), namely males; and that only under stressful conditions a post-conceptional mechanism of sex ratio determination would reduce the more fragile and higher energy-consuming embryos (generally the males) may be lost during the peri-implantation period or later. Under suboptimal conditions (epigenetic environment) the secondary sex ratio may depend on differential survival in embryogenesis, in which case mothers should produce more females, because they will have greater lifetime reproductive success with low levels of investment received.