Cytological analysis of chromosomes in the two species Parascaris univalens and P. equorum

Chromosome organization and the phenomenon of chromatin diminution in the two species Parascaris univalens (2n=2) and P. equorum (2n=4) were cytologically analysed by a variety of staining techniques (Quinacrine, Hoechst 33258, Chromomycin A₃ and C-banding). The results show that: (1) the chromosomes of the two species differ markedly in both the location and the type of heterochromatin they contain; (2) in both species there is a strong chromosome polymorphism which, however, ranges within a basic species-specific phenotype; (3) the heterochromatin can be eliminated in presomatic cells during early embryogenesis at two different stages and in both cases the consequence of this process is the generation of somatic cells with a 2n=60 karyotype. Moreover, evidence suggesting the sterility of hybrids between the two species is provided.