A comparative study of chromosomes in four species of Theodoxus (Gastropoda : Neritidae)

Karyological analysis was performed on Theodoxus baeticus (Lamarck, 1822), T. valentina (Graells, 1846), T. velascoi (Graells, 1846) and T. fluviatili s (Linne, 1758), collected from freshwater bodies of eastern Spain. Cells p ossessing diploid chromosome numbers of 2n = 25 were most common in the tis sues of males, whilst 2n = 26 was most prevalent in females. The sixth pair of chromosomes had only one homologue in mitotic cells of males, indicatin g the XO/XX sex-determining mechanism. The absolute length of chromosomes r anged from 1.65 to 6.33 mum. The relative sizes of chromosomes varied from 3.92 to 13.33% of the total haploid set length. Tn all species chromosome p airs no. 1, 4, 7-13 were composed of metacentric, pairs no. Sand 6-of subme tacentric chromosomes. Pairs no. 2 and 3 were submetacentric, subtelocentri c or submeta- subtelocentric according to the species studied. There were s ignificant differences (P< 0.05) among the relative length, as well as cent romeric indices of different chromosome pairs across the species. Karyologi cal differences were greatest between T. fluviatilis and three other Theodo xus species.