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Progenetic evolution in the deep-sea fauna

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How deep-sea worms colonise the ocean floor

Recent studies reveal the enormous biodiversity of the deep sea and the high levels of animals unique to the ocean floor. EU researchers have investigated the special type of evolution found in marine bristle worms and how this is key to their success in this environment.

Climate Change and Environment
Fundamental Research

Scientists currently possess very little information concerning the degree of genetic variation among and within deep-sea species; even basic ecological traits like growth rates and life histories are still poorly understood. This lack of knowledge regarding patterns of evolution in deep-sea environments has led to conflicting hypotheses on the origin and age of fauna living at the bottom of the ocean. The PRODEEP project addressed this knowledge gap by studying the diversity and phylogeny of Hyalinoeciin marine worms. The aim was to determine the role of progenesis within this group and understand their successful colonisation of the deep sea. Progenesis involves the retention of juvenile or even larval traits into later life by an organism through acceleration of sexual maturation. Hyalinoeciinae is a relatively small, but widely distributed monophyletic group of species. PRODEEP researcher Dr Nataliya Budaeva says: “Varying degrees of loss and reduction of traits within the subfamily enable scientists to examine the expression of likely progenetic traits in relation to ocean depth. Hence, this group of organisms was used as a model taxon of marine invertebrates to answer fundamental questions regarding the colonisation pathways of deep-sea fauna.” Why taxonomy is important Hyalinoeciin specimens from European, Australian and American natural history museums were carefully examined by project scientists. In particular, the so-called quill worms, which combine two genera, Hyalinoecia and Leptoecia. “Lack of taxonomic knowledge and insufficient sampling are the main impediments to phylogeny-based studies of the evolution of deep-sea fauna. It is therefore important to perform a thorough taxonomic revision of a group before addressing evolutionary questions,” Dr Nataliya Budaeva explains. Scientists also determined the phylogenetic position of Hyalinoeciinae species by reconstructing the first comprehensive phylogeny of the family Onuphidae, based on molecular data. “Taxonomical revision of quill worms based on morphological traits and sequence data of 6 genes, led to a discovery of 5 species new to science and the accurate re-description of 20 already known species,” states Dr Budaeva. The reconstructed phylogenetic tree was used to map morphological traits used in feeding and locomotion such as parapodia (feet) and chaetae (bristles) and obtain independent evidence concerning the absence or loss of traits in species with presumed progenetic origin. Why progenesis evolved Following complete taxonomical revision, the vertical ranges of species of quill worms were analysed. Results showed a clear break in their distribution at a depth of around 2000 meters. The larger and less progenetic Hyalinoecia were mostly present in shallower waters, while smaller and juvenile-like progenetic Leptoecia were widely distributed in the deep sea. A hypothesis was developed to explain the possible pathways of deep-sea invasion by Hyalinoeciinae. According to Dr Budaeva: “The progenetic appearance and accelerated sexual maturation in deep-sea Leptoecia could have evolved as an adaptation to seasonality of the food supply in the deep-sea high-latitude environment.” Understanding the phylogenetic relationships, diversity, morphology and life history traits in Hyalinoeciins will shed light on how this group has become so successful. It will also strengthen European understanding and competitiveness in this area of marine studies and deepen links between scientists studying the deep-sea environment in Europe and around the world.

Keywords

PRODEEP, deep-sea, evolution, progenesis, Hyalinoeciinae

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