NEXT WEEK Tuesday 15th April at 4pm, 312.207 at Curtin University
Dr Kate Trinajstic
Curtin University
Dept. of Chemistry
The evolution of sexual reproduction: an examination of fossilized copulatory structures in the earliest jawed vertebrates.

Placoderms, an extinct paraphyletic group of armoured fishes resolved near the base of jawed vertebrates, are the first vertebrates to evolve external copulatory structures and internal fertilization. In living chondrichthyans and osteichthyans copulatory structures are mostly derived from the pelvic fins. However, it has recently been recognized that in placoderms the paired male claspers are independent of the pelvic fin and located posterior to the pelvic girdle and fins. Thus claspers in placoderms and chondrichthyans develop in very different ways. In addition, preserved abdominal muscles with an unusual orientation and until now unknown function have been recovered from placoderms and we suggest that these could have extended and manipulated the claspers during mating. This suggests that the claspers in placoderms and chondrichthyans are not homologous, a hypothesis supported by recent phylogenetic analyses, and the history of vertebrate sex is more complicated than first thought.

Paired appendages, including claspers, are a combination of dermal and perichondral bone, and represent iteratively repeated structures developing along the body flank within an enlarged ‘competent stripe’ for paired fin development. The competence to form fins would have originated in the jawless vertebrates, in the Osteostraci, by the evolution of a contact between the lateral plate mesoderm and the ectoderm. We suggest that in placoderms the independent development of paired copulatory structures involved a posterior extension of the ‘competent stripes’ for fin development, which previously was thought to be limited to the region between the paired pectoral and pelvic fins. In the jawless fish Euphanerops paired anal fins, are present which suggests that this zone of fin competence was more extensive in primitive forms and the ability to project appendages from the body was more fluid than previously hypothesized.

Other new insights into early vertebrate reproduction included the discovery of multiple embryos in pregnant females, embryos of differing sizes and of different sexes (e.g. male claspers preserved in some embryos) which again suggests more complex reproductive strategies in primitive vertebrates than previously appreciated. As a result of these discoveries placoderms are considered to be the first vertebrates to have practiced intimate sexual reproduction and provide the earliest fossil evidence of vertebrate copulation.

Please join us for drinks and nibbles after the presentation.

Link to map of Bentley Campus, Curtin University: http://properties.curtin.edu.au/maps/
Please find about our coming seminars in the attached document and at: http://blogs.curtin.edu.au/science-seminars/category/environment-agriculture/
To subscribe to weekly announcements: http://blogs.curtin.edu.au/science-seminars/rss-feeds/
For queries, or if you are interested in presenting at the E&A seminars, please contact Dr Bill Bateman (Bill.Bateman@curtin.edu.au)