Insights into the evolution and development of Haliclona indistincta (Porifera, Haplosclerida)
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Abstract: Haliclona indistincta (order Haplosclerida, family Chalinidae) is an intertidal, and shallow sub-tidal, dwelling sponge found in Ireland, Britain and elsewhere that possesses a highly variable morphology. It is a member of the H. rosea group, proposed as monophyletic by de Weerdt (1986; 1989) based on a shared skeletal structure. Haliclona is a notoriously problematic genus for systematics because morphological characters are both few in number and highly plastic. This difficulty is noted for the order Haplosclerida as a whole, which is the largest and most diverse group of sponges. In recent years, additional insights into the phylogenetics of the haplosclerids, and Haliclona, from molecular data has shown incongruence with the classical classification based on morphology. Whether variable specimens assigned to H. indistincta belong to the same species and whether H. indistincta is even a haplosclerid has come into question. As with most species of haplosclerids, very little has been investigated with H. indistincta beyond the initial description of the species. This project explored the phylogenetic position of H. indistincta utilizing two independent gene loci (28S rRNA and mtCO1), to test if it belonged within the haplosclerids and if the H. rosea group is monophyletic. Congruence between morphological and molecular data was found, as both confirmed the phylogenetic position of H. indistincta within the haplosclerids although this species was more closely related to members of a different family (i.e. Niphatidae). Morphological variation was found consistent with two species morphotypes, but these were very similar genetically. The reproductive cycle and developmental biology of H. indistincta was also investigated. Utilizing histology, this species was found to be primarily hermaphroditic, however gonochorism was also observed. Data supported the hypothesis that the male gametes (i.e. spermatozoa) were derived from choanocytes and the female gametes (i.e. eggs) were derived from archaeocytes. A fusing behaviour was observed in the larvae of this species and fused larvae settled normally. Typically, the larvae went through three distinct mobile stages (signified by changes in the body shape) prior to settlement. Cellular rearrangement and differentiation was documented from the first free-swimming stages up to 32 days of settlement. Data was shown that supports the hypothesis that ciliated epithelial cells of the larvae transdifferentiate into two separate cell types of the juvenile (i.e. choanocytes and pinacocytes). Two previously undescribed cell types were documented (cells with inclusions Types 1 and 2). I suggest the cell with inclusions Type 1 have key roles in the developmental process of the species including: canal system formation (which I suggest has started forming prior to settlement), cell fate, and possibly nutrient processing. The basal apparatus of the ciliated epithelial cells were described in detail in hopes that further synapomorphies among haplosclerid species could be found. This work highlights both the need for, and benefits of complementing molecular phylogenetic studies with morphological data.
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