The hermaphrodite, colonial ascidian <I>Diplosoma listerianum </I>(Chordata: Urochordata) mates by releasing sperm that disperse to neighbours, where they fertilise eggs that have been retained internally rather than spawned. The species is able to utilise highly dilute sperm: comparison with published information on a sea urchin, which released both eggs and sperm for external fertilisation, showed that <I>D. listerianum</I> maintained comparable levels of fertilisation at sperm concentrations two or three orders of magnitude lower than the echinoderm. Laboratory clones of <I>D. listerianum</I> displayed surprisingly high levels of sexual incompatibility. Fecundities of numerous pairwise crosses varied widely and suggested a continuous scale of computability. Although correlations of computability between reciprocal crosses appeared positive, considerable noise was present in the data and some crosses showed strongly asymmetrical compatibility. Patterns of sperm precedence with a five-day mating internal showed clear initial bias towards the first of two acting males. The proportion of second-male paternity (P<SUB>2</SUB>) subsequently increased with time. Estimated P<SUB>2</SUB> for entire progeny arrays was consistently greater than 0.5, but varied widely. When mating interval was reduced, mate order effects appeared to be moderated. In competition with an alternative sperm source, acting males fathered more progeny if previously mated to a particular female than if no mating history existed, an advantage probably derived from fertilisations by stored sperm. When virgin acting female colonies were given mixtures of sperm at widely divergent concentrations, offspring were shared between the two sperm sources in approximately the ratio of each mixture. However, there existed a small but statistically significant deviation from the fair raffle model, in that sperm at the lower concentration consistently achieved a greater than expected share of paternity. Environmentally-determined fixed female preferences could be responsible for this negative frequency dependence ('rare male effect').
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:593279 |
| Date | January 2000 |
| Creators | Pemberton, A. J. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
Page generated in 0.0014 seconds