Can resource costs of polyploidy provide an advantage to sex?

Abstract

The predominance of sexual reproduction despite its costs indicates that sex provides substantial benefits, which are usually thought to derive from the direct genetic consequences of recombination and syngamy. While genetic benefits of sex are certainly important, sexual and asexual individuals, lineages, or populations may also differ in physiological and life history traits that could influence outcomes of competition between sexuals and asexuals across environmental gradients. Here, we address possible phenotypic costs of a very common correlate of asexuality, polyploidy. We suggest that polyploidy could confer resource costs related to the dietary phosphorus demands of nucleic acid production; such costs could facilitate the persistence of sex in situations where asexual taxa are of higher ploidy level and phosphorus availability limits important traits like growth and reproduction. We outline predictions regarding the distribution of diploid sexual and polyploid asexual taxa across biogeochemical gradients and provide suggestions for study systems and empirical approaches for testing elements of our hypothesis.

Figure 1

A conceptual model of how nutrient limitation contributes to the maintenance of sex. (a) Variation in bodily P content is due to variation in nucleic acid content (Elser et al., 2003, 2006). (b) High P organisms have higher intrinsic growth rates (Elser et al., 1996, 2003), but may be more susceptible to P limitation (Sterner and Elser, 2002). (c) Taxa with higher ploidy have higher per-cell nucleic acid content; they may also have higher nucleic acid content per unit mass. (d) If taxa with higher ploidy have higher bodily nucleic acid content, they should also have higher bodily P content. (e) and (f) If (b) and (d) are true, success of asexual polyploids will vary more with environmental P availability than will the success of sexual diploids. This greater sensitivity to P availability may be sufficient on its own to maintain sex when P availability is low (e), or it may simply reduce the competitive advantage of asexuals (f).