A genomic march of the penguins

It’s undeniable that penguins are a marine representative of the charismatic megafauna group. I have an affinity for stuff we need microscopes to see, BUT I agree that penguins are cute (just LOOK at these National Geographic photos…they’re even in comics). I’m guessing that many of us have also watched “March of the Penguins”, although maybe you also were today years old when you learned the original French version was narrated in first-penguin by the stars of the show themselves in “La Marche de l’Empereur”.

Our hearts all melt a tiny bit when we see a fluffy baby chick waddle around on the ice. But. Have you ever contemplated how many different penguin species there are, where exactly they’re found on the globe and how they ended up where they currently reside? If you’re like me, (and don’t work on anything remotely related to penguins), you might not be well versed in the diversity of these flightless diving birds.

In a study out in PNAS in August, led by Juliana A. Vianna, are you ready for it? “PENGUINOMES”, coined by our very own Jeremy Yoder. As Helen Zaltzman might say, try using it in an email today…I would love to keep it around.

https://twitter.com/JBYoder/status/1295783463669231617?s=20

In this study, that I admit has great figures, the authors set the stage to learn a bit more about the currently recognized 18 species of penguins found across the Southern Hemisphere. Check this article out for another summary of this research as well as some quotes from the authors themselves. While their cuteness is widely known, apparently how they have diversified and spread across their current habitats is not as well understood.

Vianna and coauthors sequenced 22 new penguin genomes to ~30x coverage using an Illumina HiSeq X platform. Additionally, they sequenced a giant petrel genome to use as an outgroup. The study then used these data to try to piece together the puzzle of penguin diversification.

“We used 22 new genomes from 18 penguin species to reconstruct the order, timing, and location of their diversification, to track changes in their thermal niches through time, and to test for associated adaptation across the genome.”

Vianna et al., PNAS 2020

According tot he study, it looks like changing climate conditions was mainly what led to penguin diversification, as it resulted in the opening of the Drake Passage (~49 to 17 million years ago) and consequently a more intense Antarctic Circumpolar Current (ACC). The ACC, by the way, is pretty darn important. Very (very very) briefly, this current flows clockwise around Antarctica, moves a ton of water very quickly, and maintains the sharp boundary between the freezing cold waters of Antarctica and the ‘warmer’ water from the subantarctic. It does a lot of other things too, for a good primer on what it does, check this out.

From their data, the authors conclude that the ancestor of current penguins likely first ranged along the coast of Australia and New Zealand (vs Antarctica). It turns out that some like it cold, since they suggest that the first diversification event was linked to the establishment of penguins in the Antarctic, while a bit later another ancestor colonized the South American Coast (okay, some also like it hot). Figure 1 is a great visual of the evolutionary history outlined in the paper, not to mention some images demonstrating some of the diversity across currently identified species. Interestingly they also found that there were likely multiple points at which introgression events occurred (Figure 2), with the rockhopper penguins harboring the majority of these events.

Figure 2. (Vianna et al., PNAS 2020) Summary of the main introgression episodes among penguin taxa.

Vianna et al. found 104 genes were under positive selection and also that a bunch of them are linked to each other functionally. They fall into two main groups of functions: one is linked to “broad cellular functions”, while the other includes functions linked to “cardiovascular activities (e.g., blood pressure, oxygen, metabolism, coagulation)”. These adaptations, they state, are important for “diving and maintenance of high core body temperatures”.

Figure 3. (Vianna et al., PNAS 2020) Analysis of genes under positive selection in extant penguin lineages.

Vianna and colleagues also performed a reconstruction of sea surface temperatures to shed light on the temperature range of ancestral and currently living penguins. The study concludes that the ACC led to the dispersal of penguins and the genomes they analyzed reveal that penguins living at different temperatures have adaptations in genes linked to dealing with thermoregulation (how an organism keeps its body temperature at juuust the right spot) and oxygen metabolism. The authors go on to suggest that the some of the changes they found in the genome could allow for greater oxygen-binding capacity and ultimately allow for deeper and longer dives in some penguins.

From the widely recognized emperor and king penguins, to the adorable gentoo and apparently quite stylish macaroni penguin (named for their yellow crest that looks a bit macaroni), the authors provide some additional insight into the origins of these interesting animals, which was only possible with the mountain of data from their recently sequenced….penguinomes.

Journal reference:

Scher, H.D. and Martin, E.E., 2006. Timing and climatic consequences of the opening of Drake Passage. science312(5772), pp.428-430.

About Kelle Freel

I'm currently a postdoc working at the Hawai'i Institute of Marine Biology with Dr. Mike Rappé. I'm interested in the biogeography and ecology of microbes, especially of the marine variety. After studying a unique genus of marine bacteria at Scripps Oceanography in grad school, I moved to France, where I worked with a group studying yeast population genomics. In my free time, I like to do outdoorsy stuff, travel, and cook.
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