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Review
. 2011 Jul 22;278(1715):2081-90.
doi: 10.1098/rspb.2011.0289. Epub 2011 Mar 23.

The Chastity of Amoebae: Re-Evaluating Evidence for Sex in Amoeboid Organisms

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Free PMC article
Review

The Chastity of Amoebae: Re-Evaluating Evidence for Sex in Amoeboid Organisms

Daniel J G Lahr et al. Proc Biol Sci. .
Free PMC article

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 macro-organisms need not apply to microbes. Eukaryotic diversity is made up of 70+ 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.

Figures

Figure 1.
Figure 1.
Distribution of amoeboid lineages in the eukaryotic tree of life. This phylogenetic hypothesis of eukaryotic evolution is adapted from Parfrey et al. [13], and depicts the well-supported higher-level groupings of eukaryotes. The lineages that have members with amoeboid morphology are in bold. Images depict exemplary amoeboid organisms and were retrieved from Micro*scope (http://starcentral.mbl.edu/microscope/portal.php). Paraphyletic lineages are indicated by (p). SAR indicates the group composed of Stramenopila, Alveolata and Rhizaria.
Figure 2.
Figure 2.
Distribution and types of evidence for sex in the main lineages of the two largest amoeboid groups: (a) Amoebozoa and (b) Rhizaria. The topology of these illustrative trees is a consensus of well-supported lineages derived from Tekle et al. [38], Pawlowski & Burki [32], Shadwick et al. [39] and Parfrey et al. [13]. Dashed lines represent non-monophyletic taxa. Black circles, confirmed sexual life cycle; grey circles, direct evidence for sex (meiosis, karyogamy or sex genes); white circles, indirect evidence for sex (cytoplasmic fusion, presence of putative gametes).

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