Threatened rattlesnakes' inbreeding makes species more resistant to bad mutations
Scientists complete first genomic analysis of Eastern massasaugas

Date:
October 4, 2021
Source:
Ohio State University
Summary:
The first look at a threatened rattlesnake species' recent genetic
history suggests that inbreeding necessitated by limited habitat
may not be as detrimental as theory would predict it to be.



FULL STORY ==========================================================================
The first look at a threatened rattlesnake species' recent genetic
history suggests that inbreeding necessitated by limited habitat may
not be as detrimental as theory would predict it to be.


==========================================================================
In fact, scientists speculate that Eastern massasauga rattlesnakes may
have pre-adapted to living in small, isolated populations -- where the
most dangerous genetic mutations that arose could be easily exposed
and purged.

Researchers sequenced the genomes of 90 Eastern massasauga rattlesnakes,
which were listed as threatened under the Endangered Species Act in
2016 because of loss and fragmentation of their wetland habitat. For comparison, the researchers also sequenced 10 genomes of a close relative,
the Western massasauga rattlesnake, a common species with no limitations
on breeding opportunities and large populations.

The Ohio State University team found that the most potentially damaging
gene mutations were less abundant in the Eastern than the Western
species. This finding suggests the breeding limitations of small, isolated populations might be accompanied by an evolutionary advantage of being
able to elbow out genetic variants that get in the way of survival,
said H. Lisle Gibbs, professor of evolution, ecology and organismal
biology at Ohio State and senior author of the study.

"This is something that has been reported very recently in other
endangered species, but it's the first time it's been shown in a reptile," Gibbs said. "We always worry about genetics and the loss of variation
and what it means to be in a small population in which there's lots of inbreeding. At least in this species, maybe it's not such a big deal.

"From a conservation perspective, perhaps we can downplay genetics
and say ecology -- such as habitat restoration -- is more important."
Gibbs completed the study with Alexander Ochoa, a former postdoctoral researcher at Ohio State who is now a postdoctoral scholar at the
University of Central Florida. The research is published in the journal Molecular Ecology.



========================================================================== Eastern massasauga rattlesnakes live in isolated spaces in midwestern and eastern North America, and evolutionary theory posits that the inevitable inbreeding in such populations threatens species with extinction as
genetic mutations accumulate. The smallest populations might reach
30 snakes, but Ohio's Killdeer Plains Wildlife Area is home to one of
the most genetically diverse and largest populations in the country,
numbering in the thousands.

Gibbs has studied Eastern massasaugas for over two decades and, as
director of the Ohio Biodiversity Conservation Partnership, advises the
Ohio Department of Natural Resources on management of the species.

"Through years and years of study, we know that most populations are
isolated, like little natural zoos scattered throughout the landscape,"
Gibbs said. "Due to habitat degradation, we've known they show little
variation -- but we've never actually looked at variation in genes
that code for things that matter to a rattlesnake." Only recently has
it been possible to apply the research techniques perfected with the
human genome to work with this species. Gibbs and Ochoa zeroed in on identifying mutations in genes that may affect survival and reproduction
to gauge how hazardous inbreeding might be to Eastern massasaugas.

Though a higher overall number of potentially deleterious mutations
were found in the common Western massasaugas, that didn't translate
to more threats to their survival because most troublesome gene copies
were offset by protective copies. That can happen only in heterozygotes,
which have two different copies, or alleles, of a particular gene -- one inherited from each parent. Because of generations of inbreeding, Eastern massasaugas are much more likely to have two copies of the same allele.



========================================================================== "That's why inbreeding has impacts -- because that's when you get two
bad alleles showing up together, with no good allele to compensate,
so there is a negative effect," Gibbs said. "There's more inbreeding,
so overall you get more mostly bad mutations together, but the really
bad ones, because they're exposed, are also eliminated at a much greater
rate." Through another analytical technique comparing the narrowing of
the Eastern and Western massasauga genetic makeup over several hundred
years, Gibbs and Ochoa confirmed the impact human activity has had on the Eastern massasauga's swampy habitat. Unlike the Eastern species, Western massasaugas live in grassy and woodland regions of the south-central
United States that are less densely populated by humans.

"We looked at what has happened in these snakes and their population
sizes over the last 300 years, which is when humans have been tromping
all over North America, impacting the landscape," Gibbs said. "The
impacts in terms of reducing population sizes are greater in Eastern
than in Western massasaugas over this period." The findings could
influence management decisions. A common conservation practice would
involve introducing snakes from a more genetically diverse population
into a highly isolated group to counter the effects of inbreeding.

But it turns out the Eastern massasauga might benefit more from
preservation of its habitat while the genetics takes care of itself.

"This counterintuitive result makes us rethink what living in a small population is, and whether genetic problems are as important as we think
they are," Gibbs said. "This is certainly not to say living in a small population isn't bad -- it just may be that the genetic effects are not
as bad as we thought." This work was supported by the State Wildlife
Grants Program administered jointly by the U.S. Fish and Wildlife
Service and the Ohio Division of Wildlife, with funds provided by the
Ohio Biodiversity Conservation Partnership between Ohio State and the
Ohio Division of Wildlife, as well as the National Science Foundation.

========================================================================== Story Source: Materials provided by Ohio_State_University. Original
written by Emily Caldwell. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Alexander Ochoa, H. Lisle Gibbs. Genomic signatures of inbreeding
and
mutation load in a threatened rattlesnake. Molecular Ecology,
2021; DOI: 10.1111/mec.16147 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211004104115.htm

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