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BMC Evol Biol
2014 Aug 28;14:170. doi: 10.1186/s12862-014-0170-7.
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Phylogeny of Myzostomida (Annelida) and their relationships with echinoderm hosts.
Summers MM
,
Rouse GW
.
Abstract
BACKGROUND: Myzostomids are marine annelids, nearly all of which live symbiotically on or inside echinoderms, chiefly crinoids, and to a lesser extent asteroids and ophiuroids. These symbionts possess a variety of adult body plans and lifestyles. Most described species live freely on the exterior of their hosts as adults (though starting life on the host inside cysts), while other taxa permanently reside in galls, cysts, or within the host''s mouth, digestive system, coelom, or gonads. Myzostomid lifestyles range from stealing incoming food from the host''s food grooves to consuming the host''s tissue directly. Previous molecular studies of myzostomids have had limited sampling with respect to assessing the evolutionary relationships within the group; therefore molecular data from 75 myzostomid taxa were analyzed using maximum likelihood and maximum parsimony methods. To compare relationships of myzostomids with their hosts, a phylogeny was inferred for 53 hosts and a tanglegram constructed with 88 associations.
RESULTS: Gall- and some cyst-dwellers were recovered as a clade, while cyst-to-free-living forms were found as a grade including two clades of internal host-eaters (one infecting crinoids and the other asteroids and ophiuroids), mouth/digestive system inhabitants, and other cyst-dwellers. Clades of myzostomids were recovered that associated with asteroids, ophiuroids, and stalked or feather star crinoids. Co-phylogenetic analyses rejected a null-hypothesis of random associations at the global level, but not for individual associations. Event-based analyses relied most upon host-switching and duplication events to reconcile the association history.
CONCLUSION: Hypotheses were revised concerning the systematics and evolution of Myzostomida, as well their relationships to their hosts. We found two or three transitions between food-stealing and host-eating. Taxa that dwell within the mouth or digestive system and some cyst forms are arguably derived from cyst-to-free-living ancestors--possibly the result of a free-living form moving to the mouth and paedomorphic retention of the juvenile cyst. Phylogenetic conservatism in host use was observed among related myzostomid taxa. This finding suggests that myzostomids (which have a free-living planktonic stage) are limited to one or a few closely related hosts, despite most hosts co-occurring on the same reefs, many within physical contact of each other.
Figure 3.
Maximum likelihood transformations for myzostomid lifestyle. Number and symbols near nodes refer to proportional likelihood estimations. An asterisk indicates nodes with an estimated proportional likelihood of >95%. Other scores are provided in order of most likely states and separated by a forward slash when applicable. See Figure 2 regarding non-italicized names.
Figure 4.
Maximum likelihood transformations for general host type (A) and host type by major clade (B). Symbols near nodes refer to proportional likelihood estimations. An asterisk indicates nodes with an estimated proportional likelihood of >95%. Symbols, scores, and non-italicized names as in Figure 3.
Figure 5.
Associations of myzostomids with their echinoderm hosts. Tanglegram of 88 associations among host (left side) and myzostomid (right side) phylogenies. Symbols near nodes refer to bootstrap (BS) and jackknife (JK) support scores, for ML and maximum parsimony (MP) analyses, respectively. Symbols, scores, and non-italicized names as in Figure 3.
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