What San Gorgonio Mountain can tell us about past and present gene flow: is alpine biodiversity under-examined?

Lab Note #4
May 15, 2014
Hey all -- I said the last post was going to be about alpine plants, or at least have plant pictures, but I got a little carried away with alpine landscapes and well...

The last landscape photo (bottom) in the my previous post (click here) is from the alpine zone on San Gorgonio Mountain in southern California, the southernmost glaciated peak in western North America during the Pleistocene Epoch. Not surprisingly, this mountain represents the southermost limit for many widespread alpine species complexes. I want to take a moment to highlight a few plants that occur on San Gorgonio Mountain in order to help you better understand why I'm REALLY interested in patterns of gene flow among alpine plant populations.

One of the dominant plants atop San Gorgonio Mountain are the two species of pine trees (Pinus), both of which crawl to the top as krummholz 'woodland' on the south facing slopes of the summit region at 11,500 feet elevation. Think about pine pollen: it gets around, right? It doesn't take a lot, but gene flow becomes more frequent across greater distances, and reproductive barriers therefore have to be stronger in order for closely related species to retain their identity (i.e., reduce levels of hybridization to prevent genetic swamping) -- a 'sky island archipelago' of high peaks doesn't seem like much of a barrier to gene flow among populations (localized stands) of limber pine, P. flexilis. Therefore, imagining a single species with a distribution like that of P. flexilis pictured below would make a lot of sense...



Now take a different example -- what if you can't disperse your seeds (or pollen) around very well? How do you maintain at least the minimum amount of gene flow among widely spaced satellite populations required to 'reign them in' and maintain species identity?! That is EXACTLY what we're trying to get at here. Obviously it is going to be highly variable for different systems occupying every available niche...



Above is a picture of a live plant atop San Gorgonio Mountain and the distribution of a southern California Transverse Range endemic Draba (Brassicaceae), D. corrugata. Also called the Southern Mountain Draba, this species is (likely) very closely related to another southern California endemic, D. saxosa which is restricted to the San Jacinto Mountains -- D. saxosa looks very similar to D. corrugata but lacks cauline leaves. Given that the San Gorgonio plants (D. corrugata) are closer geographically to the populations of D. saxosa in the San Jacinto Mountains, one has to wonder about what is maintaining gene flow among the Transverse Ranges (San Gabriel + San Bernardino Mountains) but has broken down among the Transverse and Peninsular Ranges (San Jacinto Mountains), giving rise to D. saxosa? Better question -- are the plants in the San Gabriel and San Bernardino Mountains REALLY the same species? We have GOT to take a closer look to figure it out!

Another example? How about Primulaceae...





Above is a picture of Androsace septentrionalis and also a map showing its distribution in western North America -- the species is actually circumpolar. This rock jasmine is highly self-pollinating, and either a biennial or possibly annual, which suggests relatively rapid accumulation of mutations in populations that are highly subject to genetic drift... It produces tiny seeds, OK, but I don't suspect there is a lot of gene flow between populations in the European Alps and those in the San Bernardino Mountains of southern California -- with molecular tools we can answer these kinds of questions.

Most importantly, by getting out into the field, we can make observations about local ecology that is not otherwise evident on many museum specimens collected by early surveyors -- this is the critical information we are trying to collect for a diversity of species in unrelated plant groups that grow in alpine environments across continents and country borders. Given that alpine plants might experience the most drastic changes in terms of habitat loss as a result of global climate change (particularly warming trends), this kind of baseline research is greatly needed.

On the note of local ecology, this 'baseline research' is leading to MANY new species discoveries -- I will highlight one for you, a species that occurs on San Gorgonio Mountain in southern California. This is a local endemic that will soon be described to Science and published as a new species (manuscript in press, California Fish and Game Journal), Silene "krantzii" (picture below). It happens to be only one member of a complex of species distributed throughout the American Southwest (map below is for the Silene verecunda species complex of which S. "krantzii" is a part).



The taxon with which S. "krantzii" is most often confused is S. verecunda subsp. platyota, which is a taller, greener, considerably less-glandular plant that grows at lower elevations in the San Bernardino Mountains (and elsewhere in California Mountains). A trial common garden corroborates the separation of these two taxa. The REAL kicker is that the nominate variety, S. verecunda subsp. verecunda, is likely a local endemic to the San Francisco Bay Area -- those plants are also VERY different from S. "krantzii" although they are probably more similar than the jolly green giant subsp. platyota. That range-wide distribution map (above) of a broadly circumscribed Silene verecunda isn't starting to look so hot... Turns out the molecules appear to be in agreement with ecological observations (my unpublished work right now). Let's just say that a whole heck of lot of work is needed for this complex of polyploid (tetraploid) catchflies to be sorted out! "Krantz's catchfly" is just one piece of the puzzle.

It reminds me so very much of my work with Claytonia in a somewhat freaky way... Different rocks = different plants?

If you haven't already, consider making a pledge for this critical biodiversity research on alpine plants. ANY amount will help (click here to contribute), the strength is in the masses. We are just over halfway through the fundraiser (less than two weeks left) and we are right at 50% of our 'all or nothing' goal -- please keep our momentum going! No one is charged unless we meet our full fundraising amount, so tell everyone to join in!

Feel free to ask questions about the research! I'm happy to answer.
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