Concerted evolution of a highly repetitive DNA family in eptatretidae (Cyclostomata, agnatha) implies specifically differential homogenization and amplification events in their germ cells.
In eight hagfish species, it is known that chromosome elimination occurs during early embryogenesis, and some highly repetitive DNA families, restricted to germ cells, have been isolated. One of these families, "EEEo2," has been isolated as DNA fragments by restriction enzyme analyses from Eptatretus okinoseanus and E. cirrhatus. In this study, EEEo2 sequences were isolated from germline DNA in E. burgeri, Paramyxine sheni, and P. atami using PCR methods. Sequence analysis revealed that these sequences are intraspecifically homogeneous, except in E. burgeri, and are interspecifically conserved with heterogeneity. The intraspecific sequence variability tends to decrease as the copy number increases. These results indicate that EEEo2 has evolved in a concerted manner. Moreover, an ancestral repeating motif consisting of triplicate subrepeats was deduced. These results suggest that EEEo2 arose as an initial amplification of this subrepeat and has evolved by saltatory replication. Phylogenetic analyses suggested the possibility that EEEo2 in E. okinoseanus and E. cirrhatus has been subjected to strong homogenizing forces for concerted evolution, whereas the force is weak in E. burgeri. In addition, EEEo2 in P. sheni and P. atami appear to have been incompletely subjected to these forces. Chromosomal in situ hybridization experiments revealed that EEEo2 sequences were located along almost their entire length of several heterochromatic chromosomes that are restricted to germ cells. These chromosomes are disposed to form a secondary association during the first meiotic metaphases, except in P. sheni. This chromosomal distribution may promote a concerted mode of sequence evolution in both nonhomologous chromosomes and homologous chromosomes and reflect the differential driving forces between species.
Nabeyama
M
M
Department of Biology, Faculty of Science, Toho University Miyama 2-2-1, Funabashi, Chiba, 274-8510 Japan.
Kubota
S
S
Kohno
S
S
D000818:Animals; D001483:Base Sequence; D002874:Chromosome Mapping; D019143:Evolution, Molecular; D018628:Gene Dosage; D005854:Germ Cells / Q000502:physiology*; D006196:Hagfishes / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D012091:Repetitive Sequences, Nucleic Acid; D017422:Sequence Analysis, DNA