Characterization and Evolution of Germ1, an Element that Undergoes Diminution in Lampreys (Cyclostomata: Petromyzontidae)

et al. 2022

Sci Rep

In the Japanese hagfish Eptatretus burgeri, 16 chromosomes (eliminated [E]-chromosomes) have been lost in somatic cells (2n = 36), which is equivalent to approx. 21% of the genomic DNA in germ cells (2n = 52). At least seven of the 12 eliminated repetitive DNA families isolated in eight hagfish species were selectively amplified in the germline genome of this species. One of them, EEEb1 (eliminated element of E. burgeri 1) is exclusively localized on all E-chromosomes. Herein, we identified four novel eliminated repetitive DNA families (named EEEb3–6) through PCR amplification and suppressive subtractive hybridization (SSH) combined with Southern-blot hybridization. EEEb3 was mosaic for 5S rDNA and SINE elements. EEEb4 was GC-rich repeats and has one pair of direct and inverted repeats, whereas EEEb5 and EEEb6 were AT-rich repeats with one pair and two pairs of sub-repeats, respectively. Interestingly, all repeat classes except EEEb3 were transcribed in the testes, although no open reading frames (ORF) were identified. We conducted fluorescence in situ hybridization (FISH) to examine the chromosomal localizations of EEEb3–6 and EEEb2, which was previously isolated from the germline genome of E. burgeri. All sequences were only found on all EEEb1-positive E-chromosomes. Copy number estimation of the repeated elements by slot-blot hybridization revealed that (i) the EEEb1–6 family members occupied 39.9% of the total eliminated DNA, and (ii) a small number of repeats were retained in somatic cells, suggesting that there is incomplete elimination of the repeated elements. These results provide new insights into the mechanisms involved in the chromosome elimination and the evolution of E-chromosomes.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Novel selectively amplified DNA sequences in the germline genome of the Japanese hagfish, Eptatretus burgeri

et al. 2022

Sci Rep

“…One of eliminated sequences in sea lamprey, Germ1, is composed of a somatically rare (SR) region and a truncated 28S rDNA sequence. Since the boundary between this SR region and the 28S rDNA fragment is identical to the 3' integrated sequence after the transposition of R2 retrotransposon, the SR region is a sequence derived from the R2 retrotransposon (Strange and Moore 2019). The SR region of Germ1 has been consistently mapped to major rDNA loci on nine meiotic bivalent chromosomes in germ (testis) cells, whereas in the somatic cells the SR region and 28S rDNA are restricted to a single mitotic chromosome pair (Smith et al 2009;Covelo-Soto et al 2014).…”

Section: Discussionmentioning

confidence: 99%

Novel selectively amplified DNA sequences in the germline genome of the Japanese hagfish, Eptatretus burgeri

et al. 2022

Preprint

In the Japanese hagfish Eptatretus burgeri, 16 chromosomes (eliminated [E]-chromosomes) have been lost in somatic cells (2n=36), which is equivalent to approx. 21% of the genomic DNA in germ cells (2n=52). At least seven of the 12 eliminated repetitive DNA families isolated in eight hagfish species were selectively amplified in the germline genome of this species. One of them, EEEb1 (eliminated element of E. burgeri 1) is exclusively localized on all E-chromosomes. Herein, we identified four novel eliminated repetitive DNA families (named EEEb3–6) from this species through polymerase chain reaction (PCR) and suppressive subtractive hybridization (SSH) combined with Southern-blot hybridization. EEEb3 was mosaic for 5S rDNA and SINE element. EEEb4 was GC-rich repeats and had one pair of direct and inverted repeats, whereas EEEb5 and EEEb6 were AT-rich repeats and composed of several sub-repeats. Interestingly, all but EEEb3 were transcribed in the testes, although no functional open reading frame (ORF) was found. We conducted fluorescence in situ hybridization (FISH) to examine the chromosomal localizations of EEEb3–6 and EEEb2, which was initially isolated from the germline DNA of this species. All of them were completely colocalized on all EEEb1-positive E-chromosomes. The estimation of the copy number by slot-blot hybridization revealed that (i) EEEb1–6 family members occupied 39.9% of the total eliminated DNA, and (ii) a small number of repeats were retained in somatic cells, suggesting the incomplete elimination. These results provide new insights about the mechanisms involved in the chromosome elimination and the evolution of E-chromosomes.

“…One of eliminated sequences in sea lamprey, Germ1, is composed of a somatically rare (SR) region and a truncated 28S rDNA sequence. Since the boundary between this SR region and the 28S rDNA fragment is identical to the 3' integrated sequence after the transposition of R2 retrotransposon, the SR region is a sequence derived from the R2 retrotransposon [32] . The SR region of Germ1 has been consistently mapped to major rDNA loci on nine meiotic bivalent chromosomes in germ (testis) cells, whereas in the somatic cells the SR region and 28S rDNA are restricted to a single mitotic chromosome pair [33], [34] .…”

Section: Discussionmentioning

confidence: 99%

Novel selectively amplified DNA sequences in the germline genome of the Japanese hagfish, Eptatretus burgeri

et al. 2022

Preprint

In the Japanese hagfish Eptatretus burgeri, 16 chromosomes (eliminated [E]-chromosomes) have been lost in somatic cells (2n = 36), which is equivalent to approx. 21% of the genomic DNA in germ cells (2n = 52). At least seven of the 12 eliminated repetitive DNA families isolated in eight hagfish species were selectively amplified in the germline genome of this species. One of them, EEEb1 (eliminated element of E. burgeri 1) is exclusively localized on all E-chromosomes. Herein, we identified four novel eliminated repetitive DNA families (named EEEb3–6) from this species through PCR amplification and suppressive subtractive hybridization (SSH) combined with Southern-blot hybridization. EEEb3 was mosaic for 5S rDNA and SINE element. EEEb4 was GC-rich repeats and had one pair of direct and inverted repeats, whereas EEEb5 and EEEb6 were AT-rich repeats and composed of several sub-repeats. Interestingly, all but EEEb3 were transcribed in the testes, although no functional open reading frame (ORF) was found. We conducted fluorescence in situ hybridization (FISH) to examine the chromosomal localizations of EEEb3–6 and EEEb2, which was initially isolated from the germline DNA of this species. All of them were completely colocalized on all EEEb1-positive E-chromosomes. The estimation of the copy number by slot-blot hybridization revealed that (i) EEEb1–6 family members occupied 39.9% of the total eliminated DNA, and (ii) a small number of repeats were retained in somatic cells, suggesting the incomplete elimination. These results provide new insights about the mechanisms involved in the chromosome elimination and the evolution of E-chromosomes.