Evolution Update

Evolution Update

Essential Parts of Polar Bear Y-Chromosome Identified Using Evolutionary Analyses

Irma Shaboian August 11, 2015

Researchers decode Y-chromosome of polar bear and confirm its split from ancestors before the last glacial period.

The effects of climate change have perhaps been harshest for the polar bears’ habitat. Located in the Arctic, this carnivorous bear (Ursus maritimus) can be found in Canada, Alaska, Russia, Greenland, and Norway; they are classified as a vulnerable species. Polar bears have been suggested to diverged from the brown bear, making the polar bear a “younger” species.

The availability and efficiency of genomic sequencing has bolstered the ability of scientists to determine how organisms are related. By identifying certain regions of DNA within an organism’s genome, tracing the evolutionary lineage of a species can be done.

Currently, genomic mapping of mammals has mainly focused on the X-chromosome; a sex chromosome passed both maternally and paternally to their offspring. In contrast, there has been less mapping of the Y-chromosome, a sex chromosome present in biologic males (X-Y) but not females (X-X) and passed paternally.

Scientists from Germany and the United Kingdom used computer modeling to study the sequenced Y-chromosome of male polar bears and demonstrate the mammal’s phylogeny from a paternal perspective.

To accomplish this, the researchers used two methods to identify parts of the Y-chromosome. The first approach was to use 32 known genes that are linked to other mammals and match them to genome map of a male bear. In doing so, the scientists were able to identify 23 specific regions of the polar bear’s genome considered unique to the Y-chromosome.

The second approach was to compare the genome of a female polar bear to that of a male polar bear. Accordingly, the researchers would be able to compare sequences and identify those unique to the Y-chromosome.

After having had studied billions of sequences, the research team was able to identify 1.9 million base pairs in the Y-chromosome and further identify 112 unique Y-chromosome sequence segments in high-resolution. Successfully, they were able to confirm evolutionary studies on polar bears using bioinformatics. The analyses done by the scientists confirm the already identified belief that there was a split of two distinctly Y-chromosomal lineages in polar bears, referred to as PO1.1 and PO2.

Furthermore, the research team was able to isolate the splitting of the paternal polar bears into two lineages during the Eemia interglacial period, between 130,00 and 120,000 years ago when the weather was warmer. Subsequently, the polar bear survived the late interglacial period. This suggests population structuring in polar bears; that is, their population was subdivided in some way.

Aside from contributing to the evolutionary knowledge of polar bears, the scientists were able to demonstrate an effective means of identifying sequences that are associated with the Y-chromosome. The latter contribution allows a more diverse study of evolutionary development.

Practical applications of Y-chromosome sequencing and sequence identification adopt a bioinformatics approach, as the researchers of the present study demonstrated. In doing so, paternal inheritance may be further studied and offer new insights about populations and their lineages.

DNA analyses have revealed that females traditionally have made greater genetic contributions to the human population than males had; the Y-chromosome is present only in males and passed paternally. Thus, tracing the phylogeny of humans and tracing their evolutionary history from a paternal perspective becomes a worthwhile cause. Taking this a step further, researchers can use Y-chromosome sequence data to focus on health and wellness outcomes unique to males.

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