There and back again: the dynamic evolution of panarthropod germ cell specification mechanisms
Posted on: 17 October 2025 , updated on: 21 October 2025
Preprint posted on 14 August 2025
Categories: developmental biology, evolutionary biology
What I like about this preprint:
Studying the evolution of development sometimes feels like trying to do a 1000-piece jigsaw puzzle without the original picture, just hoping that random pieces fit together. I like that this preprint takes a huge amount of existing data on a developmental mechanism, primordial germ cell specification, and puts it into a detailed phylogenetic context. It follows on from a previous piece by the same lab calling for this kind of work to define hypotheses for developmental evolution (1). This approach allows you to see the big picture and the unexpected macroevolutionary patterns that emerge.
Background:
The generation of gametes from a germline lineage is one of the most important events in biology, crucial to all sexually reproducing organisms. It is therefore quite surprising to find that there is considerable variability across the tree of life in how and when the germline is defined and segregated from the soma.
Primordial germ cells (PGCs) are the first cells in the embryo that will give rise exclusively to germ cells. Within metazoans it is generally accepted that there are two modes of PGC specification, induction and inheritance (2,3). Induction is specification that results from cell-cell interactions, the quintessential example being the mouse. Meanwhile, PGC specification through inheritance involves the segregation of maternally deposited factors that often form visible cytoplasmic condensates known as germ plasm (or germ granules), examples of which include Drosophila and C. elegans.
Historically, induction has been seen by some developmental biologists as an evolutionary novelty. However, reviews of the literature have demonstrated that induction is very common across animals, particularly in basally branching lineages (2,3).
Kao and colleagues build on previous work (2,4) aggregating literature from the last 165 years on germ cell appearance and specification, in this case focusing on the clade of panarthropods. Panarthropods are a diverse lineage which includes (among others) tardigrades, spiders, centipedes, crustaceans, and insects. The extensive data of PGC appearance combined with advances in panathropod phylogenetics allows the authors to draw conclusions about the evolution of PGC specification mechanisms.
Key findings:
The authors sorted species into three categories based on the timing at which PGCs are first identified: prior to/during blastoderm formation, prior to/during gastrulation, and after gastrulation. Evaluating these categories, in light of recent panarthropod phylogenies, the authors found that PGCs appear later in development in early branching euathropods, and there have been multiple independent transitions towards early specification.
The authors then used their temporal appearance categories to infer the potential specification mechanism. Although as the authors note, there is increasing molecular evidence for a spectrum of PGC specification mechanisms between purely inductive and purely inherited. PGCs that arise early and/or have been shown to contain cytoplasmic granules were inferred to have inherited specification while the opposite properties led to the assignment of an inductive mechanism.
Combining this information, and their panathropod phylogeny, the authors performed ancestral reconstruction. They found that induction is the ancestral specification mechanism in panarthropods and in insects. They also found that induction is the ancestral mechanism in the major insect clade, holometabola, in which a single origin of germ plasm was previously thought to have evolved. Instead, this result points to the repeated evolution of germ plasm from induction throughout holometabolous insects, demonstrating that this transition is highly dynamic (and not exclusively unidirectional).
The authors then focused on insects and whether there is a link between the evolution of long germ development, the unique mode of simultaneous specification of the body segments exemplified by Drosophila, and the evolution of germ plasm. This is based on observations that insects with germ plasm which also contain maternally-deposited axial patterning factors often exhibit long germ development. The authors found that there is a significant association between long germ development and germ plasm specification. This provides some evidence to the hypothesis that PGC specification through asymmetric, inherited factors, is associated with and may have co-opted mechanisms of asymmetric localisation of maternal axial determinants (5).
Finally, the authors note an association between germ plasm specification and the presence of the germ granule nucleator oskar. The results suggest that oskar has been co-opted multiple times for PGC specification by inheritance within insects.
Questions for the authors:
It would seem that category 2 species are the most likely to be reassigned to a different mode of specification given their intermediate temporal position. What happens to the outcome of the ancestral reconstruction if you apply a different or even random assignment of category 2 species to induction or inheritance?
Some species undergo gastrulation at a very early point in development (or at least at a low cell number). Does this make it difficult to assign categories to certain lineages? Is there enough data on the cell number at the onset of gastrulation to map this onto the phylogeny in the same way as long/short germ development?
It could also be illustrative, if there is sufficient data available, to map which species are known to utilise maternal determinants for axial patterning onto the phylogeny to get a feel for how likely the hypothesized model in Figure 4 is.
Bibliography
- Church SH, Extavour CG. Null hypotheses for developmental evolution. Development [Internet]. 2020 Apr 15 [cited 2020 Apr 28];147(8). Available from: https://dev.biologists.org/content/147/8/dev178004
- Extavour CG, Akam M. Mechanisms of germ cell specification across the metazoans: epigenesis and preformation. Development. 2003 Dec 15;130(24):5869–84.
- Johnson AD, Richardson E, Bachvarova RF, Crother BI. Evolution of the germ line–soma relationship in vertebrate embryos. 2011 Mar 1 [cited 2025 Sept 30]; Available from: https://rep.bioscientifica.com/view/journals/rep/141/3/291.xml
- Nieuwkoop PD, Sutasurya LA. Primordial Germ Cells in the Invertebrates: From Epigenesis to Preformation. CUP Archive; 1981. 282 p.
- Whittle CA, Extavour CG. Causes and evolutionary consequences of primordial germ-cell specification mode in metazoans. Proc Natl Acad Sci USA. 2017 June 6;114(23):5784–91.
doi: https://doi.org/10.1242/prelights.41699
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