On the reality and recognisability of asexual organisms: morphological analysis of the masticatory apparatus of bdelloid rotifers

Abstract

On the reality and recognisability of asexual organisms: morphological analysis of the masticatory apparatus of bdelloid rotifers. -Zoologica Scripta , 36 , 361-370. Species concepts and definitions have been a long-standing debate in evolutionary biology since before Darwin, and almost all proposed solutions are based upon grouping and clustering, with species conceived as somehow biological distinct entities, originated and maintained mainly by reproductive isolation. Lacking reproductive exchange, asexual organisms such as bdelloid rotifers, the best-supported clade of so-called 'ancient asexuals', pose an interesting challenge to debates over the reality of species. However, few data are available on bdelloid rotifers. The only evidence has been that bdelloid species have been more consistently recognised than in their sister sexual group, the monogonont rotifers, across successive taxonomic treatments, but this is confounded by the much lesser degree of taxonomic interest in bdelloids. We applied geometric morphometrics analyses on shape and size of hard masticatory pieces, named trophi, of 1420 bdelloids, belonging to 48 populations of eight traditional species, to test the hypothesis of recognisability of bdelloids. Our morphological analysis confirms that traditional bdelloid species are separated distinct entities, possessing trophi morphologies divided by gaps between taxa, similar to patterns of morphological features in sexually reproducing organisms. In common with most microscopic understudied organisms, bdelloid rotifers harbour much previously undescribed diversity: we found significant differences in trophi morphology within traditional species, revealing the existence of cryptic taxa, similar to those also found in facultatively sexual monogonont rotifers. We confirm that recognisability in bdelloids is not qualitatively different from other small understudied animals such as monogononts, and that sexual versus asexual reproduction does not lead to differences in morphological diversity patterns, as previously suggested based on interpretation of taxonomic revisions.

The importance of being a bdelloid: Ecological and evolutionary consequences of dormancy

“…Bdelloid diversity has been established upon morphology alone, as cross-mating tests are obviously not possible in organisms that do not mate. Nevertheless, different morphological features are congruent in grouping individuals; for example, geometric morphometric analyses of shape and size of trophi in more than 1400 individuals of the genus Rotaria revealed clusters which were consistent with traditional species identification, which is based on body traits other than trophi (Fontaneto et al , 2007b. Moreover, molecular phylogenetic analyses showed that entities equivalent to species actually exist in bdelloids.…”

Planarian (Platyhelminthes, Tricladida) Diversity and Molecular Markers: A New View of an Old Group

“…The Yule-Coalescent transition analysis as implemented in GMYC [13,14] is one of these methodologies. This coalescent-based method allows species delimitation by distinguishing branching patterns between interspecific (Yule model; speciation and extinction) and intraspecific (coalescence of alleles) processes on a phylogenetic tree.…”

Spatial niche partitioning in epibiont rotifers on the waterlouse Asellus aquaticus

“…The species assemblages we found on the waterlice are quite typical. The two bdelloid species of the genus Rotaria are the ones with the strictest epibiotic association with the waterlouse (May 1989); they can be rarely found free living, and when so, always in waterbodies with waterlice (Fontaneto et al 2007a). Rotaria and Testudinella do not move very much on the host and usually attach their foot to the exoskeleton of the waterlouse, becoming temporarily sessile, and feed by filtering water with their ciliated corona (Donner 1965;Koste 1978).…”

“…magnacalcarata was not influenced by the presence of other species as well, either in its localization or in its abundances. All the filter-feeder species we analyzed (i.e., R. magnacalcarata, R. socialis, and the T. caeca-T. elliptica group) potentially feed on similar resources, since their jaws have similar shape and size (De Smet 2005;Fontaneto et al 2007b), but R. magnacalcarata is much larger in body size, and this could provide a selective advantage against the others. A more efficient feeding potential may be reflected in the negative influence R. magnacalcarata has on both R. socialis and the species group T. caeca-T. elliptica, whose presences are significantly reduced when R. magnacalcarata is present on the host.…”

“…The mechanisms allowing coexistence in the same habitat for species that have similar ecological needs and therefore are expected to compete for limiting resources is an old, but central issue in ecological studies. Niche specialization allows species coexistence (Behmer and Joern 2008;Finke and Snyder 2008), but interspecific competition can be avoided also by temporal (Albrecht and Gotelli 2001;Kronfeld-Schor and Dayan 2003;Gutman and Dayan 2005) and/or spatial segregation (Schlyter and Anderbrant 1993;Boughman et al 2005;Friberg et al 2008) between potential competitors in the use of habitat resources.…”