In this chapter, we have focused on large-scale as well as the inter- and intracellular events that take place during embryogenesis and the formation of adult structures. In particular, we discussed how the adult body plan is laid down by a cascade of gene expression, and the role of cell–cell communication in development. Based on your knowledge of these topics, answer several fundamental questions:

How did we discover that selector genes specify which adult structures will be formed by body segments?

Video transcript

Hey, everyone. Let's look at our next question. It says which of the following genes determines which adult structures will be formed by each body segment. When we're thinking about these events of early embryonic development, it could be helpful to kind of recall the sequence of events, the different genes that sort of cascade in those earliest developmental stages to recall which one specifically determines the adult structures formed by each segment. So I'm going to just post a little kind of flow chart here describing these events. And as we look it over, we see that the earliest genes affecting events are the maternal effects jeans and these are a result of events that happened even prior to fertilization in the unfertilized egg. Because during egg formation, M RNA and proteins that are produced by the transcription of the maternal genome are stored in the unfertilized egg and they're often stored in positional gradients in the egg. So after fertilization, those concentration gradients of those different products begin to establish the directionality of the early embryo. So again, those maternal effects genes establish basic directionality in the embryo by means of concentration gradients. So anterior versus posterior dorsal versus ventral. So this is very early maternal effects, genes then regulate the expression of our next step. The segmentation genes and segmentation genes cause as their name indicates the segmentation of the embryo. So basic directionality, now we begin to sort of subdivide it. Um and this kind of three types there, you have gap jeans which involve the formation of the large multi segment regions. And then the expression of gap jeans affects the expression of pair rule genes. Those subdivide the embryo into what looked like stripes. Um That's the beginning of segmentation into different different segments. And then finally, the segment polarity genes established directionality within each segment. Then the segmentation genes regulate the expression of the homoerotic selector genes. And now that our embryos divided into segments, the homoerotic selector genes are what determine segment identity and then determine which structures will develop from those segments. So if something goes wrong with these, for instance, you might have legs growing out of the head where antenna should be since they determine those structural fate, structural destiny of the cells in each segment. So with that sort of order of events in mind, let's look at our answer choices. So choices, maternal effect jeans. Well, that is not what determines which adult structures. That's the earliest stage there. Ah segmentation genes, we're not there yet just dividing them into basic segments. So obviously, choice D both A and B is also wrong and we're left with choice. Cr homoerotic selector genes do that in yellow just to make it a little more clear. So the homoerotic selector genes, which do indeed determine which adult structures will be formed in each body segment. See you in the next video.