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Independently evolving species in asexual bdelloid rotifers.

Fontaneto D, Herniou EA, Boschetti C, Caprioli M, Melone G, Ricci C, Barraclough TG - PLoS Biol. (2007)

Bottom Line: Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms.Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade.The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Biologia, Università di Milano, Milan, Italy.

ABSTRACT
Asexuals are an important test case for theories of why species exist. If asexual clades displayed the same pattern of discrete variation as sexual clades, this would challenge the traditional view that sex is necessary for diversification into species. However, critical evidence has been lacking: all putative examples have involved organisms with recent or ongoing histories of recombination and have relied on visual interpretation of patterns of genetic and phenotypic variation rather than on formal tests of alternative evolutionary scenarios. Here we show that a classic asexual clade, the bdelloid rotifers, has diversified into distinct evolutionary species. Intensive sampling of the genus Rotaria reveals the presence of well-separated genetic clusters indicative of independent evolution. Moreover, combined genetic and morphological analyses reveal divergent selection in feeding morphology, indicative of niche divergence. Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms. Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade. The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

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Scheme Showing the Predicted Patterns of Genetic and Morphological Variation Underlying Our Tests of Alternative Scenarios of Diversification(A) Hypothetical trees showing expected genetic relationships among a sample of individuals under the  model that the sample is drawn from a single asexual population (H0) and under the alternative model that the clade has diversified into a set of independently evolving entities (H1).(B) Expected variation in two ecomorphological traits evolving either neutrally (H0) or by adaptive divergence (H1) in a genus that has diversified into six genetic clusters. Note that a mixed pattern is possible: Some genetic clusters may have experienced divergent selection on morphology, whereas others have not.
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pbio-0050087-g002: Scheme Showing the Predicted Patterns of Genetic and Morphological Variation Underlying Our Tests of Alternative Scenarios of Diversification(A) Hypothetical trees showing expected genetic relationships among a sample of individuals under the model that the sample is drawn from a single asexual population (H0) and under the alternative model that the clade has diversified into a set of independently evolving entities (H1).(B) Expected variation in two ecomorphological traits evolving either neutrally (H0) or by adaptive divergence (H1) in a genus that has diversified into six genetic clusters. Note that a mixed pattern is possible: Some genetic clusters may have experienced divergent selection on morphology, whereas others have not.

Mentions: Focusing on the genus Rotaria (Figure 1), one of the best-characterized genera of bdelloids, we use combined molecular and morphological analyses to distinguish alternative scenarios for bdelloid diversification (Figure 2). First, the entire clade might represent a single species, that is, a swarm of clones with no diversification into independently evolving subsets of individuals. Second, the clade may have diversified into a series of independently evolving entities. By “independently evolving,” we mean that the evolutionary processes of selection and drift operate separately in different entities [8,9], such that genotypes can only spread within a single entity. Possible causes of independence include geographical isolation or adaptation to different ecological niches [10,17]. The expected outcome is cohesion within entities but genetic and phenotypic divergence between them [9–11].


Independently evolving species in asexual bdelloid rotifers.

Fontaneto D, Herniou EA, Boschetti C, Caprioli M, Melone G, Ricci C, Barraclough TG - PLoS Biol. (2007)

Scheme Showing the Predicted Patterns of Genetic and Morphological Variation Underlying Our Tests of Alternative Scenarios of Diversification(A) Hypothetical trees showing expected genetic relationships among a sample of individuals under the  model that the sample is drawn from a single asexual population (H0) and under the alternative model that the clade has diversified into a set of independently evolving entities (H1).(B) Expected variation in two ecomorphological traits evolving either neutrally (H0) or by adaptive divergence (H1) in a genus that has diversified into six genetic clusters. Note that a mixed pattern is possible: Some genetic clusters may have experienced divergent selection on morphology, whereas others have not.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC1828144&req=5

pbio-0050087-g002: Scheme Showing the Predicted Patterns of Genetic and Morphological Variation Underlying Our Tests of Alternative Scenarios of Diversification(A) Hypothetical trees showing expected genetic relationships among a sample of individuals under the model that the sample is drawn from a single asexual population (H0) and under the alternative model that the clade has diversified into a set of independently evolving entities (H1).(B) Expected variation in two ecomorphological traits evolving either neutrally (H0) or by adaptive divergence (H1) in a genus that has diversified into six genetic clusters. Note that a mixed pattern is possible: Some genetic clusters may have experienced divergent selection on morphology, whereas others have not.
Mentions: Focusing on the genus Rotaria (Figure 1), one of the best-characterized genera of bdelloids, we use combined molecular and morphological analyses to distinguish alternative scenarios for bdelloid diversification (Figure 2). First, the entire clade might represent a single species, that is, a swarm of clones with no diversification into independently evolving subsets of individuals. Second, the clade may have diversified into a series of independently evolving entities. By “independently evolving,” we mean that the evolutionary processes of selection and drift operate separately in different entities [8,9], such that genotypes can only spread within a single entity. Possible causes of independence include geographical isolation or adaptation to different ecological niches [10,17]. The expected outcome is cohesion within entities but genetic and phenotypic divergence between them [9–11].

Bottom Line: Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms.Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade.The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

View Article: PubMed Central - PubMed

Affiliation: Dipartimento di Biologia, Università di Milano, Milan, Italy.

ABSTRACT
Asexuals are an important test case for theories of why species exist. If asexual clades displayed the same pattern of discrete variation as sexual clades, this would challenge the traditional view that sex is necessary for diversification into species. However, critical evidence has been lacking: all putative examples have involved organisms with recent or ongoing histories of recombination and have relied on visual interpretation of patterns of genetic and phenotypic variation rather than on formal tests of alternative evolutionary scenarios. Here we show that a classic asexual clade, the bdelloid rotifers, has diversified into distinct evolutionary species. Intensive sampling of the genus Rotaria reveals the presence of well-separated genetic clusters indicative of independent evolution. Moreover, combined genetic and morphological analyses reveal divergent selection in feeding morphology, indicative of niche divergence. Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms. Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade. The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

Show MeSH