The XX-XY (male-heterogametic) sex chromosome system in Polydora curiosa (Polychaeta : Spionidae)

Chromosome complements of Polydora curiosa Radashevsky, 1994 (Polychaeta: S pionidae) were examined using air-drying method. Regenerating worm tips and gamete-bearing segments were used for the analysis. A diploid chromosome n umber of 34 was found in well-spread metaphase plates of somatic cells. A p air of sex chromosomes (XY) with the Y considerably larger than the X was f ound in males. The size and shape of the Y chromosome were individually var iable. Sixteen normal bivalents and one large univalent-like bivalent were observed in metaphase I plates of spermatocytes. The large bivalent is the end-to-end association of heteromorphic sex chromosomes having very low str uctural homology. ks many as 39% of studied spermatocytes of P. curiosa wer e found to be tetraploid cells. The sex chromosome system described herein is the first case of XY male heterogamety in the Spionidae and only the sec ond one so far documented in the Polychaeta.