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Hybridisation and Diversification in the Adaptive Radiation of Clownfishes


Hybridisation and Diversification in the Adaptive Radiation of Clownfishes

Glenn Litsios et al. BMC Evol Biol.


Background: The importance of hybridisation during species diversification has long been debated among evolutionary biologists. It is increasingly recognised that hybridisation events occurred during the evolutionary history of numerous species, especially during the early stages of adaptive radiation. We study the effect of hybridisation on diversification in the clownfishes, a clade of coral reef fish that diversified through an adaptive radiation process. While two species of clownfish are likely to have been described from hybrid specimens, the occurrence and effect of hybridisation on the clade diversification is yet unknown.

Results: We generate sequences of three mitochondrial genes to complete an existing dataset of nuclear sequences and document cytonuclear discordance at a node, which shows a drastic increase of diversification rate. Then, using a tree-based jack-knife method, we identify clownfish species likely stemming from hybridisation events. Finally, we use molecular cloning and identify the putative parental species of four clownfish specimens that display the morphological characteristics of hybrids.

Conclusions: Our results show that consistently with the syngameon hypothesis, hybridisation events are linked with a burst of diversification in the clownfishes. Moreover, several recently diverged clownfish lineages likely originated through hybridisation, which indicates that diversification, catalysed by hybridisation events, may still be happening.


Figure 1
Figure 1
Cytonuclear incongruence. Majority-rule consensus tree with all compatible groups for mitochondrial (on the left) and nuclear (on the right) datasets. Red dots indicates nodes having Bayesian posterior probabilities lower than 0.95. Links are drawn to highlight the topological differences. Colours correspond to species clades (see Table 1) as follows: black: outgroups, turquoise: percula, grey: Amphiprion latezonatus and A. chrysopterus (monospecific lineages), orange: clarkii, blue: akallopisos, rose: Australian, green: ephippium, yellow: polymnus, brown: Indian. A coloured legend, which repeats this information for the major clades, is located at the right of the figure.
Figure 2
Figure 2
Homoplasy excess test. Panel (a) shows the distribution of the bootstrap support for each node which showed a homoplasy excess during the removal analysis. Each boxplot shows the null distribution of bootstrap values for the node and the red dot shows the bootstrap value of the node when the taxon of interest (name on the right) has been removed. Nodes names as given on the left correspond to that of panel (b) which shows the consensus tree of the nuclear phylogeny. There, nodes and taxa which showed up as outliers in the analysis have been coloured in orange. Species used as outgroup are shown in grey. The size of node labels has been altered to facilitate reading.
Figure 3
Figure 3
Mitochondrial phylogeny of hybrid individuals . Majority-rule consensus tree with all compatible groups for mitochondrial sequences with the inclusion of hybrid specimens in the analysis. Panel (a) shows a close-up on the topological position of hybrid specimens 1 to 3 and panel (b) of A. leucokranos. The topology of the rest of the phylogeny (not shown) is as in Figure 1.
Figure 4
Figure 4
Nuclear phylogeny of hybrid cloned sequences. Majority-rule consensus tree with all compatible groups for nuclear sequences with the inclusion of hybrid specimens in the analysis. The two panels (a & b) are close-ups on the topological position of the cloned sequences. The topology of the rest of the phylogeny (not shown) is as in Figure 1.
Figure 5
Figure 5
Distribution of potentially hybridising species . Species distribution following [17], of species likely implicated in hybridisation events. The localities of the sequenced hybrid individuals are shown by yellow dots or area when known. The precise geographical origin of the hybrid 2 individual is unknown, the question mark sign indicates the general area where it was collected.

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