The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms

Abstract

Amoebae are generally assumed to be asexual. We argue that this view is a relict of early classification schemes that lumped all amoebae together inside the 'lower' protozoa, separated from the 'higher' plants, animals and fungi. This artificial classification allowed microbial eukaryotes, including amoebae, to be dismissed as primitive, and implied that the biological rules and theories developed for macroorganisms need not apply to microbes. Eukaryotic diversity is made up of 70þ lineages, most of which are microbial. Plants, animals and fungi are nested among these microbial lineages. Thus, theories on the prevalence and maintenance of sex developed for macro-organisms should in fact apply to microbial eukaryotes, though the theories may need to be refined and generalized (e.g. to account for the variation in sexual strategies and prevalence of facultative sex in natural populations of many microbial eukaryotes). We use a revised phylogenetic framework to assess evidence for sex in several amoeboid lineages that are traditionally considered asexual, and we interpret this evidence in light of theories on the evolution of sex developed for macro-organisms. We emphasize that the limited data available for many lineages coupled with natural variation in microbial life cycles overestimate the extent of asexuality. Mapping sexuality onto the eukaryotic tree of life demonstrates that the majority of amoeboid lineages are, contrary to popular belief, anciently sexual, and that most asexual groups have probably arisen recently and independently. Additionally, several unusual genomic traits are prevalent in amoeboid lineages, including cyclic polyploidy, which may serve as alternative mechanisms to minimize the deleterious effects of asexuality.

Commentary on the chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms

“…As Lahr et al [1] point out the absence of the observation of sex is not the same as absence of sex. If one learns to 'think about sex all the time,' one may be more likely to find it.…”

“…In a way, it is too bad that biologists, in general, do not spend enough time thinking critically about sex as a biological phenomenon. Lahr et al [1], in their review of sex in amoeboid eukaryotes, make a good start at calling attention to this problem.…”

“…Nonetheless, given how often they have been observed, many of the simplest, most reduced amoebae may be truly asexual (see [1]). Even some lineages of more complex amoebae, such as some of the protosteloid amoebae that have been observed intensively [9], may lack sex.…”

“…Therefore, if we can understand the origins of asexual lineages, it may tell us something about whether alternatives to sex are necessary to 'protect' long-lived asexual lineages from such perceived risks as Muller's Ratchet (see [1]). …”

“…It is a phenomenon that is exclusive to eukaryotes. Lahr et al [1] elegantly define the essential components of sex; they are ' . .…”