Gender bender

The unique sex life of the malarial parasite

Published: Thursday 31 August 2000

malarialthe malarial parasite Plasmodium comes in two sexes -- male and female. The different sexes, known as gametocytes, are formed in the vertebrate host -- for instance in humans. When bitten by a mosquito, both sexes are transmitted to the insect, within which fertilisation takes place.

But before that happens, a single male gametocyte gives rise to eight male gametes -- the equivalent of sperm. The female gametocyte, however, produces just one female gamete -- the equivalent of an egg. This imbalance partially redresses an earlier imbalance: at the gametocyte stage, the sex ratio in the parasite is strongly biased in favour of the female sex.

Why might this be so? The speculation, originating in a brilliant idea of the evolutionary biologist William Hamilton, has long been that female-biased sex ratios are found whenever all the young come from the same parents and have the chance to mate with each other. Because one male can mate with a large number of females, under those conditions the optimal sex ratio consists of one male to many females.

Unfortunately, there are not many examples known in which an individual can modulate its sex ratio in this fashion. What Richard Paul and his colleagues at the Pasteur Institute in Paris have now shown is that the parasite can adjust the numbers of the two sexes it produces ( Science , Vol 287, p128-131).

This adjustment is in response to environmental conditions. Specifically, as the host starts fighting back by producing a larger number of red blood cells, the parasite starts to shift its sex ratio in the direction favouring males. The shift, induced as it is by the host, reduces the overall reproductive efficiency of the parasite. The implication of these experiments supports Hamilton's theory. It suggests that the decision to differentiate into more female gametocytes than male gametocytes under normal conditions is adaptive, that is, is an evolved outcome of natural selection for reproductive success.

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