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Heredity/Evolution English
DEMYSTIFYING THE ORIGINS AND FUNCTIONS OF SEX CHROMOSOMES
2018-03-14 00:00:00
Cedric Dent

Dennis Venema, a well-known biologist, published a book entitled, Adam and the Genome, and much of the content relates directly to findings of studies published by other researchers after the release of his book. In particular, the book briefly talks about Y-Chromosome Adam and Mitochondrial Eve, which is not an actual reference to the Biblical characters so much as an allusion to the origins of the X and Y chromosomes. A new study published in Communications Biology on March 8 elucidates the provenance of male-female differentiation as a concept, focusing on the evolution of sex chromosomes, finding these origins to be surprisingly unsophisticated. Adding to the lack of sophistication, another study conducted by experts at Linköping University debunks the conjecture that the X chromosome factors significantly into the differences in male and female lifespans.

By and large, people agree, regardless of their perspectives, that humanity all traces back to some singular source overall. Whether you accept Darwinism or not or only in part, you likely agree that your idea of the source, for lack of a better word, applies to everyone. As such, this typically means we can agree that there s a common, universal ancestor. Delving only slightly further, most would even venture to say that there is a common male ancestor and a common female ancestor, which is what this X- and Y-chromosome origin issue is about. The Biblical Adam and Eve references are usually just one of many points of contention that start there, but it speaks to a question that persists regarding the true origin of mankind.

In the chromosomal sense, the real question is whether or not this Adam and Eve coexisted or might have existed separately as members to a much broader populace responsible for the majority of our genome. In the former case, they would likely be the progenitors of our entire genome together whereas, in the latter case, they would independently have been the sources of only our mitochondrial DNA and Y chromosomes. Such is the general trajectory of the debate, and our autosomes (non-sex chromosomes) would be attributed to that broader populace in the latter case.

Venema doesn’t really address the question in his book but, rather, glosses over the debate to make the seemingly more important argument that Mitochondrial Eve and Y-Chromosome Adam could have both existed concurrently and not have been the Biblical characters their retroactively given names imply. Even as he dodges the bulk of the debate, it’s an argument worth making, and it would then simplify things if found to be true. One might be inclined to simply dismiss this and say that there’s no way it’s that simple, but sex chromosomes aren’t proving to be very complex lately.

The researchers who published the Communications Biology study concentrated their efforts on two particularly insightful, multicellular volvocine species found in the Eudorina and Yamagishiella genera. These genera collectively relate to the known transition to male-female differentiation from what is known as isogamy — the lack of sex chromosomes. Eudorina and Yamagishiella colonies look very similar to one another, both bearing 32 discernible cells, yet Yamagishiella is actually isogamous whereas Eudorina has male and female gametes. The research team determined mating types for Yamagishiella by subjecting its chromosomal regions to high-throughput genome sequencing, and they discerned male-female differentiation for Eudorina. Then, they compared those regions from both.

The theory of evolution technically is, at least in part, predicated on the notion that there should be greater genetic complexity to Eudorina than for other volvocine species, but the opposite was actually proven by the results of the study. Eudorina was found to be genetically the simplest sex-determiner of its species, which means that the most important contrast between male and female Eudorina might be little more than having or not having one gene: MID. It sits in a minuscule chromosomal region. “This new study punches a hole in the idea that increased genetic complexity of sex chromosomes accompanied the origin of sexes,” according to the lead researcher, Umen.

“Moreover, the work also has practical implications since it expands our understanding of how to identify mating types and sexes in new species of algae that we might want to breed as crops for improved traits relating to biofuel or biotechnology applications,” Umen added. This level of simplicity suggests that we may also be attributing more things to sex chromosomes than actually makes sense, especially X chromosomes. Mitochondrial DNA being as important as it is, we often can’t help but figure it has something to do with just about everything. It only seems fitting, for example, that it would have major bearing on lifespans. At the very least, many have long pontificated that the presence or absence of an X chromosome might explain differences in male and female lifespans.

Males average shorter lifespans, but the research team in Sweden from Linköping University published findings in Evolution that suggest this isn’t because of males having one fewer X chromosome. Mind you, this isn’t only a trend in human beings but, rather, one observed in virtually all mammalian species. Across each such species, females average longer lives. Not only that, for other species in which the males have two copies of the X chromosome — different types of fish, birds, insects, and reptiles, for example — the males also average longer lifespans. This is what gives life to the so-called “unguarded X hypothesis.”

“The fruit fly is a useful model organism to test the hypothesis,” according to Martin Brengdahl, a Ph.D. student at LiU in the Physics, Chemistry and Biology Department. He says this because “females generally live longer, and the X chromosome constitutes as much as one-fifth of the genetic material. If the theory is true, the effects should be clear in the fruit fly.” In other words, that’s where it’s most observable. So, they redistributed X chromosomes for two groups of male and female fruit flies.

“There was no difference in average lifespan between females that had been inbred for the X chromosome and normal females. Inbreeding of one autosomal chromosome, on the other hand, did have a negative effect and the effects were equally large in females and males,” Brengdahl explained. This all suggests that X and Y chromosomes both might be less sophisticated than we make them out to be, which can only be surmised as a scientifically ironic commentary on the likelihood that we make more than we should of differences between sexes in general, some say.

[researchpaper 리서치페이퍼=Cedric Dent 기자]


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