Germline

The former typically occurs in plants seeds, while the latter tends to be seen in nematodes, as well as certain species of reptiles, birds, and fish.

For example, many varieties of citrus,[11] plants in the Rosaceae and some in the Asteraceae, such as Taraxacum, produce seeds apomictically when somatic diploid cells displace the ovule or early embryo.

[12] In an earlier stage of genetic thinking, there was a clear distinction between germline and somatic cells.

Plants and basal metazoans such as sponges (Porifera) and corals (Anthozoa) do not sequester a distinct germline, generating gametes from multipotent stem cell lineages that also give rise to ordinary somatic tissues.

[16] Another recent theory suggests that early germline sequestration evolved to limit the accumulation of deleterious mutations in mitochondrial genes in complex organisms with high energy requirements and fast mitochondrial mutation rates.

[20] Among humans, about five percent of live-born offspring have a genetic disorder, and of these, about 20% are due to newly arisen germline mutations.

Then primordial germ cells of the mouse undergo genome-wide DNA demethylation, followed by subsequent new methylation to reset the epigenome in order to form an egg or sperm.

The first phase, starting at about embryonic day 8.5, occurs during PGC proliferation and migration, and it results in genome-wide loss of methylation, involving almost all genomic sequences.

[25] The second phase occurs during embryonic days 9.5 to 13.5 and causes demethylation of most remaining specific loci, including germline-specific and meiosis-specific genes.

[26] At embryonic day 13.5, PGC genomes display the lowest level of global DNA methylation of all cells in the life cycle.

While undergoing mitotic expansion in the developing gonad, the male germline starts the re-methylation process by embryonic day 14.5.