When finding a mate is difficult, self-fertilization offers a tempting solution by increasing the number of offspring an individual can produce. But although “selfing” provides a stopgap solution when mates are scarce, it is frequently an evolutionary dead end; when environmental conditions change, species with high selfing rates seem prone to extinction. In an article in the June issue of GENETICS, Matthew Hartfield and Sylvain Glémin provide a glimpse into the mechanisms behind selfing’s shortcomings by examining how this reproductive strategy might limit the capacity of a species to adapt.  

A species adapts when a helpful gene variant spreads through the population by natural selection until it becomes the only version of the gene: the beneficial allele is then “fixed” in the population. To better understand this process in highly-selfing species, the researchers calculated the likelihood that a beneficial allele will fix in a species already undergoing selection at another locus.

In their mathematical model, the researchers considered two possible scenarios: that one helpful mutation may be lost if a second, even more beneficial mutation is already present at a different locus, and that an existing beneficial allele may be replaced by a second mutation that confers greater fitness. They found that in a highly-selfing species, the loss of a new helpful allele had a more powerful effect than did the replacement of the first allele. To interpret the finding, they turned to results from their own and others’ prior research, pointing out that selfers should have not only an initial increase in fecundity but should also benefit from escaping an effect known as Haldane’s sieve.

Haldane’s sieve predicts that in diploid organisms, a recessive helpful allele is less likely to fix than a dominant one. In outcrossing populations, the relative dearth of homozygotes means a rare recessive allele is less exposed to selection than a dominant one. But self-fertilization dramatically increases the number of homozygous sites, exposing recessive alleles to selection and relieving this limitation. Selfers should thus be able to reap more benefits from recessive mutations that are helpful but rare.

Despite this hypothetical advantage, the longer stretches of homozygosity in highly-selfing species also mean that the effective recombination rate is lower. In effect, the genome is less thoroughly shuffled between each generation. The researchers calculated that since these homozygous tracts stretch over longer genetic regions in highly selfing species, they increase the chance the second mutation will be lost. This is because the second allele must recombine to a genetic background carrying the first allele, which is less likely with high homozygosity. The results showed that this loss outweighs the advantages of sidestepping Haldane’s sieve, especially if new mutations are not as favorable as the one that gave rise to the first beneficial allele.

Once a species becomes self-fertilizing, it’s difficult to evolve back into an outcrossing species. Selfing, it seems, is a short-term and often perilous evolutionary path to take. The mutations contributing to self-fertilization become more and more prevalent simply because organisms bearing them make more copies of the mutated genome. To the detriment of the species as a whole, these selfish genes are difficult to dislodge from the species’ DNA. With the insights provided by this research, we are one step closer to understanding how selfing can ultimately cripple populations, one of the many examples of natural selection gone awry.

CITATION:

Hartfield, M.; Glémin, S. Limits to Adaptation in Partially Selfing Species.
GENETICS, 203(2), 959-974.
DOI: 10.1534/genetics.116.188821
http://www.genetics.org/content/203/2/959

, ,

Nicole Haloupek is a freelance science writer and a recent graduate of UC Berkeley's molecular and cell biology PhD program.

View all posts by Nicole Haloupek »

Read more in

  • Ed Grell at his dissecting microscope using ether to anesthetize his flies.

    In Memoriam: Ellsworth Herman Grell (1932–2023), a pioneer of Drosophila genome engineering and annotation

    Ellsworth (Ed) Grell blessed the Drosophila community through three enduring legacies: as a pioneer of chromosome mechanics, as a primary organizer and synthesizer of genetic knowledge in Drosophila, and as a graceful mentor to those fortunate to have known him personally. Ed grew up in rural Nebraska, completed his undergraduate studies at Iowa State, and…

  • Congratulations to the #Fungal24 Poster Award winners!

    We are pleased to announce the recipients of the GSA Poster Awards for posters presented at the 32nd Fungal Genetics Conference! Undergraduate and graduate student members of GSA were eligible for the awards, and a hard-working team of judges made the determinations. Congratulations to all! Felicia Ebot Ojong, The University of Georgia My research is focused…

  • Poster presentation tips for TAGC 2024

    You’ve been selected to present a poster at The Allied Genetics Conference 2024 in March—you’ve celebrated, made plans to attend, now what? This is an exciting opportunity to showcase your research and engage with fellow members of the genetics community, so you want to make sure you’re prepared. We wanted to offer you some tips…

  • Maximize your TAGC 2024 experience 

    A guide to all that National Harbor & DC have to offer Are you joining us for The Allied Genetics Conference 2024 in March? Make the most of your #TAGC24 experience in National Harbor! We know the science will keep you busy, but you deserve to unwind and have some fun, so we’ve curated a…

  • Early Career Leadership Spotlight: Sarah Petrosky

    We’re taking time to get to know the members of the GSA’s Early Career Scientist Committees. Join us to learn more about our early career scientist advocates. Sarah PetroskyMultimedia SubcommitteeUniversity of Pittsburgh Research Interest I am interested in understanding adaptation that has been happening recently in populations by dissecting the ways that genes underlying an adaptation…

  • TAGC 2024 Early Career Award Winners

    GSA is pleased to announce the winners of the early career awards presented at The Allied Genetics Conference 2024. These awards are specific to particular TAGC communities and recognize early career scientists’ outstanding work on their respective research organisms. The awardees will present their talks in keynote sessions at TAGC 2024.  Don’t miss the opportunity…

  • Preeminent geneticists recognized with revamped GSA Awards

    In 2022, GSA’s Board of Directors launched an audit to review the five major awards conferred by the Society. Today, we are thrilled to announce the recipients of the reimagined GSA Awards, including the new Genetics Society of America Early Career Medal. The scientists honored this year are recognized by their peers for their outstanding…

  • Fly Board funds outreach programs to spread the word about Drosophila research

    In 2020, the Fly Board voted to use part of its reserve fund to support efforts to increase trainee participation as well as equity and diversity in the Drosophila community. An awards committee decides how the money will be spent each year, and from 2020–2022, the committee posted a very broad call for applications from…

  • New members of the GSA Board of Directors: 2024–2026

    We are pleased to announce the election of four new leaders to the GSA Board of Directors: 2024 Vice President/2025 President Brenda Andrews Professor, University of Toronto It’s an honor to continue my association with the Society by serving as Vice President of the Board of Directors. I have broad knowledge of the ongoing activities…

  • A rose-ringed parakeet, one of three parrot species used in the study.

    Parrot plumage study aids breeders and endangered natural populations

    Yellow coloration is maladaptive in the wild but prized by breeders. People have long been fascinated with birds, which exhibit one of the widest ranges of coloration among vertebrates. Parrots, in particular, have captivated humans by their ability to mimic human speech and spectacular plumage. Brightly colored feathers are used primarily to attract mates, intimidate…

  • Early Career Leadership Spotlight: Sarah Gilmour

    We’re taking time to get to know the members of the GSA’s Early Career Scientist Committees. Join us to learn more about our early career scientist advocates. Sarah GilmourMultimedia SubcommitteeStowers Institute for Medical Research Research Interest Questions of evolution have always fascinated me. I am extremely fortunate to be starting out my research career in a…