Editing Germ cell (section)

==Specification==
There are two mechanisms to establish the germ cell lineage in the [[embryo]]. The first way is called preformistic and involves that the cells destined to become germ cells inherit the specific germ cell determinants present in the [[germ plasm]] (specific area of the cytoplasm) of the egg (ovum). The unfertilized egg of most animals is asymmetrical: different regions of the cytoplasm contain different amounts of [[Messenger RNA|mRNA]] and proteins.

The second way is found in mammals, where germ cells are not specified by such determinants but by signals controlled by zygotic genes. In mammals, a few cells of the early embryo are induced by signals of neighboring cells to become [[Germ line development#Germline development in mammals|primordial germ cells]]. Mammalian eggs are somewhat symmetrical and after the first divisions of the fertilized egg, the produced cells are all [[totipotent]]. This means that they can differentiate in any cell type in the body and thus germ cells. Specification of primordial germ cells in the laboratory mouse is initiated by high levels of bone morphogenetic protein (BMP) signaling, which activates expression of the transcription factors Blimp-1/[[PRDM1|Prdm1]] and Prdm14.<ref name="Saitou">{{cite journal | vauthors = Saitou M, Yamaji M | title = Primordial germ cells in mice | journal = Cold Spring Harbor Perspectives in Biology | volume = 4 | issue = 11 | pages = a008375 | date = November 2012 | pmid = 23125014 | pmc = 3536339 | doi = 10.1101/cshperspect.a008375 }}</ref>

It is speculated that induction was the ancestral mechanism, and that the preformistic, or inheritance, mechanism of germ cell establishment arose from [[convergent evolution]].<ref>{{cite journal | vauthors = Johnson AD, Alberio R | title = Primordial germ cells: the first cell lineage or the last cells standing? | journal = Development | volume = 142 | issue = 16 | pages = 2730–2739 | date = August 2015 | pmid = 26286941 | pmc = 4550962 | doi = 10.1242/dev.113993 }}</ref> There are several key differences between these two mechanisms that may provide reasoning for the evolution of germ plasm inheritance. One difference is that typically inheritance occurs almost immediately during development (around the [[blastoderm]] stage) while induction typically does not occur until gastrulation. As germ cells are quiescent and therefore not dividing, they are not susceptible to mutation.

Since the germ cell lineage is not established right away by induction, there is a higher chance for mutation to occur before the cells are specified. Mutation rate data is available that indicates a higher rate of germ line mutations in mice and humans, species which undergo induction, than in C. elegans and Drosophila melanogaster, species which undergo inheritance.<ref>{{cite journal | vauthors = Whittle CA, Extavour CG | title = Causes and evolutionary consequences of primordial germ-cell specification mode in metazoans | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 23 | pages = 5784–5791 | date = June 2017 | pmid = 28584112 | pmc = 5468662 | doi = 10.1073/pnas.1610600114 | bibcode = 2017PNAS..114.5784W | doi-access = free }}</ref> A lower mutation rate would be selected for, which is one possible reason for the convergent evolution of the germ plasm. However, more mutation rate data will need to be collected across several taxa, particularly data collected both before and after the specification of primordial germ cells before this hypothesis on the evolution of germ plasm can be backed by strong evidence.