RECENT CONTRIBUTION OF GENETICS TO THE STUDY OF SPONGE SYSTEMATICS AND BIOLOGY

Abstract

In this short review we will discuss the recent contributions of genetics to our understanding of the biology and evolution of sponges, particularly on the reappraisal of longstanding beliefs held by sponge taxonomists. The main questions addressed are the following. Are sponges animals? After centuries of controversy, there seems to be a consensus, now, that sponges are metazoans. Phylogenetic studies also indicate that animals are closely related to choanoflagellates. This indicates that choanoflagellate-like structures should not be considered a synapomorphy of the Porifera. Is the phylum Porifera monophyletic? Three main hypotheses are still prevailing: the Porifera are monophyletic; the Porifera are paraphyletic with the Hexactinellida being considered the more basal group of sponges, mostly because of their syncytial nature, or the Demospongiae and the Hexactinellida together, the Calcispongia being a sister-group of the Eumetazoa. Are the currently accepted Classes supported by molecular data? Molecular data confirms the presence of two monophyletic clades within the Calcispongia. On the other hand, the distinction of demosponge classes Tetractinomorpha and Ceractinomorpha, based on an oviparous versus viviparous reproduction, has been rejected by all molecular phylogenies produced so far. Are there true cosmopolitan sponge species? All putative cosmopolitan sponges species have turned out to be, under molecular scrutiny, groups of evolutionary very distinct species. We believe, thus, that the number of true cosmopolitan sponges is likely to be very small. Can sponge populations be homogeneous over large areas? Most sponge species studied to date have shown a rather small capability for long-range dispersal. This indicates that sponge larvae, both from viviparous and oviparous species, do not disperse very much. How important is asexual reproduction in the establishment and maintenance of sponge populations? Molecular markers confirm the presence of extensive asexual reproduction in sponges. The possibility of larval fusion and chimerism has important evolutionary consequences, but has not yet been tested molecularly.