Tuesday, June 8, 2010

Why Are there NO Bat Stars (Patiria miniata) in Wash. & Oregon?? What does genetics tell us?

Today, a 2009 paper from Carson Keever and others from UC Davis and Mike Hart's lab in UC Davis from the journal Evolution! (click to see) about understanding the history of populations of the familiar intertidal/shallow-water Bat Star, Patiria (formerly Asterina) miniata!

We start with a mystery.

If you look at the entry for bat stars in any intertidal guide to marine invertebrates, the range for this species is reliably cited as "Southern California up to British Columbia and Alaska..." But what often gets left out, is that there's a strange disjunction or absence of these starfish along a segment of the coast between Oregon and Washington!
The first step was to look at the genetic relationships between the bat stars along the coast.
Is the disjunction the start of new distinct species (i.e., isolated populations) ?? We've seen this kind of method applied in a previous post about the color morphs of Pisaster (see here).

The authors extracted DNA from 423 individuals from 14 locations from Alaska, the Queen Charlotte Islands, Vancouver Islands, and California (from Fort Bragg to Santa Barbara).

DNA from these specimens can be analyzed and compared so that they essentially assess the overall similarity between them and among them.
Plus, they can use DNA to determine how the genetic material of the sampled animals flows between and among populations. They can boil this down to a hand-dandy intuitive diagram that shows where the genetic populations of bat stars on the Pacific Northwest coast occur relative to one another...

(Fig. 1 from Keever et al., 2009)

The circles represent the samples taken. The larger size of circles represents the number of specimens sampled. The colors correspond to distinct haplotypes (the aforementioned genetic material) from each original region.
Thus, red corresponds to specimens originally from California, green from Vancouver, and blue from Alaska/northern Haida Gawaii (in the northern part of the Queen Charlotte Islands).

Each circle (i.e. the samples) has multiple colors, suggesting some mixing of populations in each region. You can see distinctly different colors (i.e., the haplotypes) between the more northern populations [i.e., blue from Alaska and North Vancouver (Haida Gawaii)] versus the red/green/blue populations in the southern regions (California and Vancouver Island).

The genetic "break" between the Northern populations versus the Southern populations may have been caused by past oceanographic barriers, such as the west to east North Pacific Current.

The North Pacific Current may form a barrier to the larval forms of this species as it effectively keeps the two populations apart..

On the other side of the range...
Based on the data on gene flow, it turns out there's a LOT of genetic mixing between southern populations, especially between California and southern Vancouver.

(modified Fig. 3 from Keever et al., 2009)

So, what do we know about the OR/WA disjunction??

There was no direct evidence that these genetic populations showed isolation or the beginning of distinct species across the Washington/ Oregon Disjunction (i.e., the no bat star zone). In fact there was 10 TIMES higher the rate of gene flow across the range disjunction relative to the gene flow in the Queen Charlotte Sound.
(Note that a high rate of gene flow generally suggests coalesence of a population rather then a separation).

So, from a genetics standpoint, the WA/OR disjunction is NOT the starting point for two separate species.

Is it glaciation?? During the last ice age, glaciers extended out to sea and could have affected the distribution of marine life. Could this have been responsible for the distribution we observe?
It turns out..no. The authors rule out glaciation as a possible mechanism.

Unfortunately for this idea-the genetic structure of Patiria miniata populations don't match up with the disjunction.

Without a clearer explanation, the authors can only speculate that there was a "recent local extirpation" which would have broken up Patiria's range along the coast....perhaps something more ecological then historical or geological. At this point, the Washington-Oregon disjunction remains a matter of "well, we know its not that..."

But stay tuned! you never know what will come up next!

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