Concerted Evolution of a Highly Repetitive DNA Family in Eptatretidae (Cyclostomata, Agnatha) Implies Specifically Differential Homogenization and Amplification Events in Their Germ Cells

Abstract

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.

Whole chromosome elimination and chromosome terminus elimination both contribute to somatic differentiation in Taiwanese hagfish Paramyxine sheni

“…In this species, more than 70% of the total DNA is expelled from presumptive somatic cells (Shichiri et al 1997). Two eliminated elements, EEEo2 and EEEb2, were previously detected as major and minor components of eliminated DNA, respectively (Nabeyama et al 2000;Kubota et al 2001). In this study, we identified four additional repetitive DNA elements accumulated as eliminated materials in P. sheni, designated EEPs1 to EEPs4.…”

“…Slot blots were obtained by spotting a dilution series of denatured DNAs on membrane filters (GeneScreen Plus, DuPont/NEN Research Products) for subsequent hybridization under a procedure described previously (Nabeyama et al 2000). The 3′-or 5′-end DIG-labeled oligonucleotide probes were prepared for this study.…”

“…HinfI fragments were blunted with T4 DNA polymerase before ligation. Selection of the recombinant clones, preparation of DNA from the screened colonies, and sequencing were performed as described previously (Nabeyama et al 2000). All nucleotide sequences were aligned with CLUSTAL W (Thompson et al 1994).…”

Circadian Rhythms in Locomotor Activity of the Hagfish, Eptatretus burgeri VI. The Effects of Cutting the Spinal Cord

“…For example, it has been reported for all hagfish investigated that predetermined chromosomes and DNA fragments are eliminated during a stage of cell division in somatic cells, but not in germ-line cells. The phenomenon, called chromosomal elimination, has not been observed in any other vertebrates (Kohno et al, 1986;Kubota et al, 1992Kubota et al, , 1993Kubota et al, , 1997Kubota et al, , 2001Kohno, 1998;Nabeyama et al, 2000;Kloc and Zagrodzinska, 2001). The functions, roles, mechanisms, and evolutionary origin of this strange phenomenon are unclear.…”

Classification and molecular organization of satellites elucidated by phylogenetic network analysis – examples from Triturus salamanders and Palorus beetles

“…Current research focuses on their structural and functional relevance (Koch 2000;Lee et al 2000), the underlying evolutionary processes (Lee et al 1997;Nabeyama et al 2000) and their use as taxonomic (King and Cummings 1997;Fedorov et al 1999) and phylogenetic markers (Árnason et al 1992;Garrido-Ramos et al 1999). Available data are mostly sequences for single, i.e., monomeric units.…”