PETROV, A.*; HOOGE, M.; TYLER, S.; Univ. of Maine, Orono: Ultrastructure of sperms in Acoela and its concordance with molecular systematics
The sperms of the Acoela, a group of lower flatworms, are filiform cells with two flagella incorporated into the cell body. Their axonemes can variously have 9+2, 9+1, or 9+0 patterns of microtubules; and singlet microtubules in the cell body can be arranged in axial or cortical positions. A hypothesis of phylogenetic relationships of acoels based on molecular characters (18S rDNA sequence data) showed that these patterns of microtubules, where known, fell into discrete monophyletic groups. To test this hypothesis, we have expanded the database of sperm characters by examining the ultrastructure of a further 10 species representing 4 acoel families. As expected, the Convolutidae falls into two unrelated groups: “small-bodied convolutids” (Convoluta pulchra, Praeconvoluta tigrina, Pseudaphanostoma smithrii) having 9+0 axonemes and cortical microtubules, and “large-bodied convolutids” (including Wulguru cuspidata) having 9+0 axonemes and axial microtubules. Also as expected, a member of the Mecynostomidae (Paedomecynostomum bruneum) has 9+1 axonemes and axial microtubules. Members of a family that appears intermediate by molecular characters, the Otocelididae, significantly have a variety of patterns: either axial (Philocelis brueggemanni) or cortical microtubules (Notocelis gullmarensis) or a combination of both (Otocelis sandara) and axonemes with both 9+2 and 9+0 patterns (N. gullmarensis) or just 9+2 (the rest). Dakuidae (Daku woorimensis) is also in this intermediate group, having 9+2 axonemes and axial microtubules, while a fifth otocelidid (Stomatricha hochbergi) has sperm characters like those of the “large-bodied convolutids” (9+0 axonemes and axial microtubules). Characters of sperm morphology generally support the molecular hypothesis of relationships and confirm a suspected polyphyly of the families Convolutidae, Otocelididae, and Actinoposthiidae. (Supported by NSF Grant No. 0118804)
