RxPG News Feed for RxPG News

Medical Research Health Special Topics World
  Home
 
   Health
 Aging
 Asian Health
 Events
 Fitness
 Food & Nutrition
 Happiness
 Men's Health
 Mental Health
 Occupational Health
 Parenting
 Public Health
 Sleep Hygiene
 Women's Health
 
   Healthcare
 Africa
 Australia
 Canada Healthcare
 China Healthcare
 India Healthcare
 New Zealand
 South Africa
 UK
 USA
 World Healthcare
 
   Latest Research
 Aging
 Alternative Medicine
 Anaethesia
 Biochemistry
 Biotechnology
 Cancer
 Cardiology
 Clinical Trials
 Cytology
 Dental
 Dermatology
 Embryology
 Endocrinology
 ENT
 Environment
 Epidemiology
 Gastroenterology
 Genetics
 Gynaecology
 Haematology
 Immunology
 Infectious Diseases
 Medicine
 Metabolism
 Microbiology
 Musculoskeletal
 Nephrology
 Neurosciences
 Obstetrics
 Ophthalmology
 Orthopedics
 Paediatrics
 Pathology
 Pharmacology
 Physiology
 Physiotherapy
 Psychiatry
 Radiology
 Rheumatology
 Sports Medicine
 Surgery
 Toxicology
 Urology
 
   Medical News
 Awards & Prizes
 Epidemics
 Launch
 Opinion
 Professionals
 
   Special Topics
 Ethics
 Euthanasia
 Evolution
  Reproduction
 Feature
 Odd Medical News
 Climate

Last Updated: Oct 11, 2012 - 10:22:56 PM
Reproduction Channel

subscribe to Reproduction newsletter
Special Topics : Evolution : Reproduction

   EMAIL   |   PRINT
Is Sex Necessary for Evolution?

Mar 26, 2007 - 7:59:12 AM , Reviewed by: Dr. Himanshu Tyagi
"By showing that asexual organisms have diverged into independently evolving and distinct entities, the researchers argue, this study refutes the idea that sex is necessary for diversification into evolutionary species."

Key Points of this article
Bdelloids have remained such an enduring enigma in part because biologists are still debating whether species exist as true evolutionary entities.
Traditional taxonomy relies on morphological differences to classify species, but it can't distinguish whether such differences reflect physical variations among a group of clones or adaptations among independently evolving populations.
With no interbreeding to maintain cohesion, the thinking goes, asexual taxa might not diversify into distinct species.
 
Bdelloid Rotifers
These microscopic invertebrates—widely distributed in mosses, creeks, ponds, and other freshwater repositories—abandoned sex perhaps 100 million years ago, yet have apparently diverged into nearly 400 species. Bdelloids (the “b” is silent) reproduce through parthenogenesis, which generates offspring with essentially the same genome as their mother from unfertilized eggs. Biologists have yet to find males, hermaphrodites, or any trace of meiosis—the process that creates sex cells—challenging the long-held assumption that evolutionary success requires genetic exchange.
[RxPG] If you own a birdbath, chances are you are hosting one of evolutionary biology's most puzzling enigmas: bdelloid rotifers. These microscopic invertebrates -widely distributed in mosses, creeks, ponds, and other freshwater repositories -abandoned sex perhaps 100 million years ago, yet have apparently diverged into nearly 400 species. Bdelloids (the "b" is silent) reproduce through parthenogenesis, which generates offspring with essentially the same genome as their mother from unfertilized eggs. Biologists have yet to find males, hermaphrodites, or any trace of meiosis- the process that creates sex cells - challenging the long-held assumption that evolutionary success requires genetic exchange.

The genetic variation created by meiosis and fertilization, theory holds, bolsters a species's capacity to weather shifting environmental conditions or resist rapidly evolving parasites. (During meiosis, the genome splits in two, and chromosome pairs swap bits of their DNA; during fertilization, the sex cells fuse to restore the complete genome.) Many multicellular eukaryotes pass through a sexual and asexual phase in their life cycle. But eschewing sex altogether, à la bdelloids, is not theoretically consistent with a long-lived evolutionary life span or extensive species diversification.

In a new study, Diego Fontaneto, Timothy Barraclough, and colleagues developed new statistical techniques for combined molecular and morphological analyses of rotifers to test the notion that species diversification requires sex. The researchers show that, despite an ancient aversion for interbreeding, bdelloids display evolutionary patterns similar to those seen in sexually reproducing taxa. How they have avoided the pitfalls of a lifestyle widely regarded as evolutionary suicide remains an open question.

Bdelloids have remained such an enduring enigma in part because biologists are still debating whether species exist as true evolutionary entities. And if they do, what forces determine how they diverge? Traditional taxonomy relies on morphological differences to classify species, but it can't distinguish whether such differences reflect physical variations among a group of clones or adaptations among independently evolving populations. In the traditional view of species diversification, interbreeding promotes cohesion within a population-maintaining the species -and barriers to interbreeding (called reproduction isolation) promote species divergence. With no interbreeding to maintain cohesion, the thinking goes, asexual taxa might not diversify into distinct species.

Fontaneto et al. defined species as independently evolving, distinct populations (or units of diversity) subject to distinct evolutionary mechanisms. They predicted that if factors other than interbreeding - such as niche specialization -controlled species cohesion and divergence, then asexual taxa should diverge along the same lines as sexually reproducing organisms. And if this were the case, they would expect to find genetic and morphological cohesion within independently evolving populations and divergence between them.

To detect independently evolving populations, the researchers analyzed marker genes isolated from clones of bdelloids collected from diverse habitats around the world. They constructed evolutionary trees using both mitochondrial and nuclear DNA sequences (the molecular "barcode" cox1and 28S ribosomal DNA sequences, respectively) to identify species within the samples. For the morphological analysis, they measured the size and shape of the rotifers' jaws (called trophi).

The morphological results largely fell in line with traditional taxonomic classifications for most bdelloid species. And species identified as related on the DNA trees typically had similar morphology. The correspondence between the molecular and morphological results suggests that the majority of traditionally identified bdelloid species are what's known as monophyletic- individuals in the same species assort together on the evolutionary tree and share a common ancestor. Only two of these traditional, monophyletic species showed significant variation in trophi size or shape among the populations; both also showed significant divergence in the DNA trees.

Using statistical models to determine the likely origin of the observed DNA tree branching patterns, the researchers show that these distinct monophyletic genetic clusters represent independently evolving entities (rather than variations within a single asexual population). But what caused them to evolve independently? Are they geographically isolated populations that evolved under neutral selection, or did they evolve into ecologically discrete species as a result of divergent selection pressures on trophi morphology?
Is Sex Necessary for Evolution?
Scanning electron micrographs showing morphological variation of bdelloid rotifers and their jaws. Have these asexual animals really diversified into evolutionary species? (Image: Diego Fontaneto)

If bdelloids have experienced divergent selection, the researchers explain, they would expect to see high variation in trophi traits between species, and low intraspecies variation (compared to neutral changes). And that's what they found -bdelloids have experienced divergent selection on trophi size (and to a lesser degree, on trophi shape) at the species level.

Altogether, these results show that the asexual bdelloids have indeed experienced divergent selection on feeding morphology, most likely as they adapted to different food sources found in different niches. By showing that asexual organisms have diverged into "independently evolving and distinct entities," the researchers argue, this study "refutes the idea that sex is necessary for diversification into evolutionary species." They hope others use their approach to study mechanisms underlying species divergence in sexual taxa to clarify the hazy nature of species and biological diversity.


Original research article: http://biology.plosjournals.org/perlserv/?request=get-pdf&file=10.1371_journal.pbio.0050099-L.pdf 
DOI of the scientific paper: http://dx.doi.org/10.1371/journal.pbio.0050099 
Publication: Gross L (2007) Who Needs Sex (or Males) Anyway? PLoS Biol 5(4): e99 
On the web: www.plos.org 

Advertise in this space for $10 per month. Contact us today.


Related Reproduction News
Artificial human sperm could make men redundant: experts
Is Sex Necessary for Evolution?
Infection Status Drives Interspecies Mating Choices in Fruit Fly Females
Why Does Sex Exist?
Declining Human Fertility is Evolutionary Adaptation
Genetic quality of sperm worsens as men get older
Songbirds boost size of eggs when hearing sexy song
Fish have menopause, study determines
Dancing ability determines mate quality
How sense of smell affects mating and aggression

Subscribe to Reproduction Newsletter

Enter your email address:


 About Dr. Himanshu Tyagi
This news story has been reviewed by Dr. Himanshu Tyagi before its publication on RxPG News website. Dr. Himanshu Tyagi, MBBS is the founder editor and manager for RxPG News. In this position he is responsible for content development and overall website and editorial management functions. His areas of special interest are psychological therapies and evidence based journalism.
RxPG News is committed to promotion and implementation of Evidence Based Medical Journalism in all channels of mass media including internet.
 Additional information about the news article
Who Needs Sex (or Males) Anyway?

Synopsis by Liza Gross

Citation: Gross L (2007) Who Needs Sex (or Males) Anyway? PLoS Biol 5(4): e99 doi:10.1371/journal.pbio.0050099

Published: March 20, 2007

Copyright: © 2007 Public Library of Science. This is an open-access article distributed under the terms of the Creative Commons Attribution License
 Feedback
For any corrections of factual information, to contact the editors or to send any medical news or health news press releases, use feedback form

Top of Page

 
Contact us

RxPG Online

Nerve

 

    Full Text RSS

© All rights reserved by RxPG Medical Solutions Private Limited (India)