Complex organization of satellite DNA library in the root-knot nematodes Meloidogyne chitwoodi and M. fallax

Satellite DNAs (satDNAs) are a predominant component of eukaryotic genomes. The study of satDNAs in closely related species contributes to the understanding of satellite DNA evolution and dynamic processes that structure eukaryotic genomes. Major satellite DNAs in two closely related nematode species, Meloidogyne chitwoodi and M. fallax have been characterized previously (1, 2). High sequence similarity has been detected among the satellite from M. fallax and one satellite subfamily from M. chitwoodi. In this work, we studied evolution and organization of these sequences in the genomes of the two species. Results reveal a pool of satellite subfamilies common to both of them, in accordance with the « library » model of satDNA evolution. This model predicts that related species share common satellite families, conserved in nucleotide sequences but amplified in differing abundances. Detailed analyses of long satellite arrays from both species revealed tracts composed of monomers of different satellite subfamilies organized in tandem together with inserted segments of unrelated sequence. Observed results show complex organization of satellite subfamilies in the form of higher order repeats (HORs). These HORs could be the origin of expanded tandem arrays based on a single subfamily and/or the place of creation of novel satellite repeats based on extensive rearrangements between different subfamilies of satellite monomers. In addition, search in Expressed Sequence Tags (ESTs) database from parasitic nematodes revealed significant similarity of the analysed HOR sequence with mRNA of unknown function from a cDNA library of M. chitwoodi. Obtained results suggest the transcriptional activity of satellite sequences in both M. chitwoodi and M. fallax species, which may be the basis of their persistence in both genomes.