Embryogenesis: an experiment that went too well

How is it possible that cnidarians evolved so many different ways to acomplish the same thing? Unipolar and multipolar ingression, invagination, delamination, epiboly… All the paths lead to ‘planula’ – a torpedo bilayered swiming larva.The term is ‘equifinal development’ and it concerns interspecific and also intraespecific variations, which are quite common among cnidarians and sponges. Meaning that embryos from the same species may gastrulate through different modes depending on mechanical forces that take place and not genetic instructions (Nakanishi et al. 2008; Y. A. Kraus and Markov 2017). Some researchers have been proposing, indeed, that the phylotipic stage works as a stabilization period in development (Richardson, 1998; Duboule, 1994). A convergent force within the embryonic development of each individual. But how this mechanism evolved? Could the pre-bilaterian planula be the first bauplan stabilization program to evolve?

Well, it might as well be the second! The first being the sessile zoophyte stage that came before any embryonic process and made it possible for embryogenesis to evolve as a pre-metamorphic experiment. If we accept Cavallier-Smith’s theory that the first metazoa was a choanoflagelate-like colony that attached to the substrate to become a filtering zoophyte (Cavallier-Smith, 2017), then it becomes natural to conceive embryogenesis emerging at the larval pre-metamorphic period of life and wihtout an imediate impact towards the animals final bauplan (i.e. the zoophyte). Making it an enviroment with low selective pressure.

In the article ‘The arising of embryogenesis from a controlled and experimental window of life‘ that will be published soon, we explore how lecitothrophy plays a major role in provoking the first experiments of cell rearrangement together with the evolution of signaling pathways presenting a great amount of crosstalk. Both of which were crucial for embryogenesis to emerge as a unique modus operandi in nature: Cells that are capable of using qualitative communication systems to transform molecular/mechanical asymetries into rapid morphological chages (i.e. Morhpogenesis!)

Burried as soon as metamorphosis came by, that was the destiny for the processes of morphogenesis that were experimented during the pre-metamorphic perioed of life. Yes, embryogenesis made its debut in a tricky scenario. But being burried by metamorphosis didn’t mean to be lost. Generation after generation, cell communication became more sophisticated and the embryo’s internal world, more complex. A battle was in motion, that is for sure… between embryonic shapes and the zoophyte filtering platform. And eventually embryogenesis would champion! Or did the battle was just transfered elsewhere? Early embryogenesis vs phylotypic stage? Is it possible that the planula stage evolved as a kind of metamorphosis within embryogenesis? What about more complex phylotypic stages? Those are questions for the near future, but one thing is for sure; it is becoming increasingly clearer that embryogenesis is something completely different than the accumulation of evolutive processes.