tag:blogger.com,1999:blog-792905364979351710.post2562963948841180591..comments2024-01-08T23:51:39.869-05:00Comments on Moss Plants and More: Two Structures, One Set of GenesJessica M. Budkehttp://www.blogger.com/profile/15186781052879876123noreply@blogger.comBlogger3125tag:blogger.com,1999:blog-792905364979351710.post-75108108895326964242013-09-19T00:54:54.390-04:002013-09-19T00:54:54.390-04:00Thanks for the comments. I'm glad you enjoyed ...Thanks for the comments. I'm glad you enjoyed the post! I also like thinking about homology and it always seems to result in an engaging discussion. When I think about structural homology the criterion I use is that structures in the present organisms have evolved from a common ancestor that had an ancestral structure that gave rise via descent with modification to both of the present structures. I acknowledge that it is a bit short-sighted of a definition because it does not take into account any of the underlying genetics. Overall I think that definitions of homology really depend on the level at which they are examined and the criteria that are used. If we used a genetic cascade criterion, we could define homologous structures as those controlled by the same genetic network, which would shake up my evaluation of the rhizoid/root hair homology.<br /><br />Despite the alternative, I would still say that rhizoids and root hairs are non-homologous structures, since they did not evolve from a common ancestor with a structure that resulted in the two via descent with modification. That being said, considering it is the same genetic cascade expressed in a different location controlling their development, they do have quite a lot in common. What could we call that? Maybe genetic homology, rather than structural homology? Or loosen up the homology definition so that both types of criteria could fit in? I think that the lack of consensus is one reason why homology is still a topic for debate. <br /><br />I agree that the co-opting of genetic networks from the gametophyte to the sporophyte generation is probably more common of a phenomenon than people might think. On one hand it is not very surprising that the genetic programs and the structures that they control could be moved from one generation or one location to the other, but I always find it amazing when science can actually demonstrate their commonalities. I bet you are right that it depends on you speciality science background whether you think of these processes as more likely or not. <br /><br />Thanks for your thoughts!Jessica M. Budkehttps://www.blogger.com/profile/15186781052879876123noreply@blogger.comtag:blogger.com,1999:blog-792905364979351710.post-66126478208781509262013-09-18T23:53:05.350-04:002013-09-18T23:53:05.350-04:00Michael says it much better than I was going to.Michael says it much better than I was going to.Rosie Redfieldhttps://www.blogger.com/profile/06807912674127645263noreply@blogger.comtag:blogger.com,1999:blog-792905364979351710.post-37694054987653526512013-09-16T22:02:28.258-04:002013-09-16T22:02:28.258-04:00It is a very interesting story and I like your pre...It is a very interesting story and I like your presentation of it. The homology question is something I think about a lot, and I know that my opinion is colored by my having a background more in genetics than in comparative morphology. <br />Personally, I am not currently comfortable saying that rhizoids and root hairs are non-homologous. They emerge from different structures, so they are technically non-homologous structures, but is that a meaningful way of thinking if the same genetic cascade is just triggered in a different place? If 1) the last common ancestor had rhizoids and 2) such a filamentous structure is adept at nutrient absorption, it would not be surprising that it was repositioned as the body plans were altered. In this case, both this family of bHLH proteins and auxin is upstream of both rhizoids and root hairs, but two datapoints don’t prove that they are conserved regulatory networks beyond any doubt. <br />As for the gametophyte/sporophyte difference, I sense that people often overestimate the difficulty in moving some aspect from one to the other generation and others (usually molecular biologists) tend to dismiss the difficulty too easily. Clearly, many developmental programs have switched generations, but it was probably only possible as part of a dramatic, concerted change that shifted whole networks to being active in the other generation. Given what we know about genetic cascades, perhaps a few specific regulatory gene changes facilitated the developmental change. (I believe some of Stefan’s transcriptome papers discussed conservation of generation-specific gene expression patterns and found that more homologs were either non-generation-specific or had different specificities in moss versus Arabidopsis than were same-generation-specific in both.)<br />Thanks for the good read!Mike Priggehttps://www.blogger.com/profile/04295504817688133474noreply@blogger.com