HIGHLY REPETITIVE DNA-SEQUENCES PROVIDE EVIDENCE FOR A LACK OF GENE FLOW BETWEEN 2 MORPHOLOGICAL FORMS OF HERDMANIA-MOMUS (ASCIDIACEA, STOLIDOBRANCHIA)

The genomic structure of two morphological forms of the pyurid ascidia n Herdmania momus - H. momus forma curvata and H. momus forma grandis - are compared using a rapidly evolving highly repetitive element isol ated from the genome of H. momus forma curvata. A 663 bp (base pair) C ia I satellite sequence is present only in the genome of H. momus form a curvata, and hence can be used as a form-specific marker. Low-string ency Southern blot analyses, using the Cla I satellite as a probe, rev ealed that the genomes of H. momus forma grandis and another pyurid as cidian, Pyura stolonifera, do not contain similar sequences to the H, momus forma curvata repetitive element. In addition to this genomic di ssimilarity, there are a number of significant reproductive and develo pmental differences between the two H. momus forms, and interform fert ilisation rates are significantly lower than intraform rates. The mole cular, reproductive and developmental differences between H. momus for ma curvata and H. momus forma grandis indicate the presence of strong barriers to gene flow.