"Since I cannot prove a lover, To entertain these fair well-spoken days, — I am determined to prove a villain"

One of the first things that we have learned in our developmental biology class is that development is hierarchical. Which is to say that a cell that becomes a liver cell will not change back to anything else. Moreover, the cells that derive from that cell will also be liver cells, and they too cannot reverse their fate. This is a story about hierarchical determination in a quite extreme form.

Copidosoma floridanum is one of the strangest animals around. It is a parasitoid wasp, animals that are normally compared to the Hollywood monster “Alien”, but frankly, this is far stranger. The adult is free living, and flies around laying eggs in moth caterpillars. One of those eggs will then divide rapidly inside the moth, forming not one but many, many larvae. Most of those larvae will begin the mundane parasitoid task of devouring the moth from the inside out while it is still alive. But some larvae will develop early and gain bodies developed for swimming and fighting. These larvae move through their host’s body, seeking out and killing any other parasitoids that they find. They ensure the survival of their sisters by wiping out everyone else. This “soldier caste” of larvae will never make it out of the moth; they die before adulthood, sacrificing themselves for the benefit of their clone siblings (fig. 1).

Fig 1: These pictures show the two types of larvae. Soldier caste on the left, reproductive caste on the right. From Donnell et al. 2004.

Researchers have done substantial research on this species and other so-called “polyembryonic parasitoids” on account of their unmitigated badassery, as well as smaller matters such as the interesting biological questions that they pose. Questions such as: “How do the two castes (reproductive and soldier) develop from a single genetically identical cell in a environment that is completely uniform, i.e. a single host body?”

Donnell et al. determined the answer to that question by following the inheritance of germ cells (cells that would form eggs and sperm) throughout the development of this wasp. First they identified a gene that is only active in germ cells, called vasa, and then created a florescent antibody to the gene product of vasa. They could then follow the cells in Copidosoma floridanum that will become eggs and sperm. As it turns out, germ cells are all derived from only one of the cells in the four-cell embryo (fig. 2). This asymmetry continues throughout development. When the embryo splits into a bunch of smaller blobs, each of which will develop into a larva, only some of those blobs will have germ cells. The ones that do will be in the reproductive caste, and the ones that do not will be soldiers (fig. 3).

In the four-celled stage, only one cell will produce all of the germ cells for all of the reproductive adults. This cell contains a florescent die that makes it appear red and it is pointed out by a white arrow. From Donnell et al. 2004.

Once the embryo is being partitioned into clumps of cells that will each develop into a larva, the germ cells are more widely spread out. Most clumps have some germs cells (again they are the glowing red ones) and they will develop into the reproductive caste (labeled SM). The clumps that do not have germ cells are going to grow into soldiers (PS) or are already starting to develop into soldiers (S). From Donnell et al. 2004.

This paper seems to provide a lovely example of hierarchy in development. The cells that will develop into germ cells are already partitioned off by the four-cell stage. No other cells will become germ cells and none of the cells derived from the first germ cell will be anything else. The asymmetrical partitioning of those germ cells then allows for the differentiation of different castes when it could not be accomplished with genetic or environmental cues. And again, these divisions are final; if you do not get any germ cells you are doomed to a brief, violent life and an early death. No other options are possible, if none of your cells came are derived from that one special cell in the four-cell embryo, your fate is sealed, and you will never grow your wings, and you will never see the sky.

The quote in the title is from Shakespeare's Richard III.  



Donnell D, Corley L, Chen G, Strand M. 2004. Caste determination in a polyembryonic wasp involves inheritance of germ cells. Proceedings of the National Academy of Sciences of the United States of America 101: 10095.