Why is sex good?

November 22, 2010 • 5:57 pm

by Greg Mayer

And by sex, I mean, of course, “… the union (SYNGAMY) of two genomes, usually carried by gametes, followed some time later by REDUCTION, ordinarily by the process of meiosis and gametogenesis” (Futuyma, 2009:388). Most of the organisms we know and love– oak trees, lobsters, goldfish, cats–  reproduce sexually. But a few of our favorite organisms– whiptail lizards prominent among them– reproduce asexually.

Cnemidophorus inornatus (sexual ancestor), C. neomexicanus (unisexual daughter species) , C. tigris (sexual ancestor). c Alistair J. Cullum. Used with permission.

At first glance, what the asexual whiptails are doing makes complete evolutionary sense: why bother producing unproductive males, when you can double your reproductive output by having nothing but daughters?  If we start a sexual population with one male and one female, and suppose that females on average have four surviving offspring, two of whom will be female, then the population increases from 2 to 4 to 8 to 16 to… etc. If we start with two asexual females, who also average four surviving offspring, all of whom are female, the population increases from 2 to 8 to 32 to 128 to… etc. You can see that asexuals reproduce a lot faster than sexuals. And it wouldn’t matter if the population wasn’t increasing– the asexuals would come to constitute a higher and higher proportion of the total population. This reproductive advantage of asexuality is called the cost of sex (google image that term for an interesting mix of scientific and non-scientific illustrations!).

So if sex has such a high reproductive cost, why are so many organisms sexual?  This is where the whiptails are revealing. Tod Reeder,  C.J. Cole, and Herb Dessauer, in their 2002 review of Cnemidophorus evolution, found that

the capability of instantly producing parthenogenetic clones through one generation of hybridization has existed for approximately 200 million years, yet the extant unisexual taxa are of very recent origins. Consequently, these lineages must be ephemeral compared to those of bisexual taxa.

Indeed, the asexual whiptails have evolved so recently that the ancestral sexual forms can in most cases be readily identified (see figure 6 in Reeder et. al). That asexual taxa are of recent origin appears to be true for animals in general (with some notable exceptions):  asexuality appears to be an evolutionary dead end. This implies that there is some long term advantage to sexuality, so that asexual species do not prosper and diversify, but rather are extinguished. The paucity of asexuals, despite their large reproductive advantage, argues for a short term advantage to sex as well. There have been a number of suggestions, most supposing that sex is advantageous in fluctuating or changing environments, so that sexual lineages would have higher fitness than asexual lineages within a population.

An essay by Matt Ridley posted at the PBS website for their Evolution series of a few years ago considers some of these issues, as does this page by someone at Brown University, and Nature has an open article collection on the subject. Two of the classic introductions to the subject are Sex and Evolution by George C. Williams, and The Evolution of Sex by John Maynard Smith.

Hybridization and parthenogenesis in whiptail lizards

November 19, 2010 • 12:26 am

by Greg Mayer

Not much in the way of culinary pleasures here. (Although Jerry’s piece on the Inquisition killed my appetite, anyway). Reader Pete Moulton asked for some references on hybridization and parthenogenesis in whiptail lizards (Cnemidophorus [or Aspidoscelis] and related teiid lizards), in particular C. (A.) uniparens.

Desert grassland whiptail, by Davepape from Wikipedia.

A. uniparens is a triploid unisexual. It resulted from a cross of two bisexual species (A. inornata [mother] and A. burti [father]), which produced a diploid unisexual, which then backcrossed to inornata to produce the triploid uniparens. The unisexuals reproduce clonally, i.e. offspring are exact genetic copies of their mothers, except for new mutations. Courtship and ‘pseudocopulation’ between parthenogenetic females promotes reproduction. The situation is summarized nicely by Cole et al. (2010):

The natural origin of diploid parthenogenesis in whiptail lizards has been through interspecific hybridization. Genomes of the parthenogens indicate that they originated in one generation, as the lizards clone the F1 hybrid state. In addition, hybridization between diploid parthenogens and males of bisexual species has resulted in triploid parthenogenetic clones in nature. Consequently, the genus Aspidoscelis contains numerous gonochoristic (= bisexual) species and numerous unisexual species whose closest relatives are bisexual, and from whom they originated through instantaneous sympatric speciation and an abrupt and dramatic switch in reproductive biology.

The selection of papers below includes both classics and recent papers, with a preference towards ones where online full text was available (see pdf links below). These papers are all about whiptails of the family Teiidae. Laurie Vitt and Jana Caldwell, in their fine text Herpetology (Academic Press 2009), record about 50 species of  parthenogenetic lizards (adding in a few they missed) in eight families (including the whiptails), and one species of parthenogenetic snake.

Wright, J.W. and C.H. Lowe. 1968. Weeds, polyploids, parthenogenesis, and the geographical and ecological distribution of all-female species of Cnemidophorus. Copeia 1968: 128-138. no pdf (A classic on unisexual ecology.)

Parker, E.D. and R.K. Selander. 1976. The organization of genetic diversity in the parthenogenetic lizard Cnemidophorus tesselatus. Genetics 84:791-805. pdf (A classic on unisexual genetics.)

Crews, D. and K.T. Fitzgerald. 1980. “Sexual” behavior in parthenogenetic lizards (Cnemidophorus). Proceedings of the National Academy of Science USA 77: 499-502. pdf (A classic on unisexual behavior.)

Reeder, T.W., H.C. Dessauer, and C.J. Cole. 2002. Phylogenetic relationships of whiptail lizards of the genus Cnemidophorus (Squamata, Teiidae) : a test of monophyly, reevaluation of karyotypic evolution, and review of hybrid origins. American Museum Novitates 3365:1-62. pdf (In this paper, the genus Aspidoscelis is resurrected for part of the genus Cnemidophorus; because there is such a huge literature under the name Cnemidophorus prior to 2002, both names must be used when searching the literature. The part on hybrid origin begins on page 25.)

Cole, C.J., L.M. Hardy, H.C. Dessauer, H.L. Taylor, and C.R. Townsend. 2010. Laboratory hybridization among North American whiptail lizards, including Aspidoscelis inornata arizonae × A. tigris marmorata (Squamata: Teiidae), ancestors of unisexual clones in nature. American Museum Novitates 3698:1-43. pdf

The American Museum of Natural History’s Digital Library has pdf’s of all the Museum’s publications, and it has been a center for studies of parthenogenetic lizards. More papers can be found by going to the Digital Library site and searching on ‘Cnemidophorus’, ‘Aspidoscelis’, and ‘parthenogenesis’.

UPDATE. The numbers of parthenogenetic species of lizards and snakes compiled by Vitt and Caldwell and given above refers only to obligately (or nearly so) parthenogenetic species, not facultatively parthenogenetic ones (like Komodo dragons, boa constrictors, and some other snakes; they have a separate discussion of the facultative species in their book).