Irvine, CA, USA. Molly Burke shows that where sex is involved, evidence of evolution in more than 500 genes can be linked to a variety of traits, including sexual maturation, size, and life span, indicating a gradual, widespread network of selective adaptation.

A team at the University of California, Irvine (UCI) deciphered how fruit flies, bred to rapidly develop and reproduce, actually evolve over time. Most people don’t think of flies as close relatives, but previous research established that humans and other mammals share 70 percent of the same genes as the tiny, banana-eating insect known as Drosophila melanogaster.

The findings, reported in the journal Nature, contradict the long-held belief that sexual beings evolve the same way simpler organisms do and could fundamentally alter the direction of genetic research. “This research really upends the dominant paradigm about how species evolve,” said ecology and evolutionary biology professor Anthony Long, the primary investigator.

Drosophila is a genus of small flies that belong to the family Drosophilidae. Family members are often called fruit flies (but include vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies).

Drosophila melanogaster, in particular, has been a model research organism for nearly a century. The species has a relatively short short life span (about two weeks) and a genomic structure that has a number of parallels to human beings.

While genetics led the way with initial uses for D. melanogaster, it has become an important investigative vehicle for developmental biology.

Researchers study how a complex organism can arise from a relatively simple fertilised egg. There is a great deal of work being done on how various adult structures develop in the pupa (such as the compound eye, wings, legs and other organs).

The drosophila egg is about half a millimeter long.

• It takes roughly one day after fertilisation for the embryo to develop and hatch into a worm-like larva.

• The larva eats and grows continuously, moulting one, two, and four days after hatching (1st, 2nd and 3rd instars).

• After two days as a third instar larva, the larva moults one more time to form an immobile pupa.

• Over the next four days, the body is completely remodelled to give the adult winged form, which then hatches from the pupal case and is fertile within about 12 hours.

Timing is for 25°C; at 18°C, development takes twice as long.

mutation

DNA

Michael Rose is a professor of ecology & evolutionary biology. “This is actually decoding the key DNA in the evolution of aging, development and fertility,” said ecology & evolutionary biology professor Michael Rose, who joked that the flies “live fast and die young.”

Lead author and doctoral student Molly Burke (cf. lead image) compared super flies, originally bred in the Michael Rose laboratory, to a control group on a genome-wide basis, the first time such a study of a sexually reproducing species has been done.

The work married DNA “soup” gathered from the adapted flies with cheap, efficient technology that uses cutting-edge informatics tools to analyze the DNA of entire organisms.

“It’s really exciting,” she said. “This is a new way of identifying genes that are important for traits we’re interested in — as opposed to the old hunting and pecking, looking at one gene at a time.”

Based on that old and flawed paradigm, Rose noted, treatment schemes have been knowledge-limited, including some medications for general treatment and those for specific diseases, with serious side effects.

He said those side effects probably occur because researchers were targeting single genes, rather than the hundreds of possible gene groups like those Burke found in the flies.

Scientists who did not participate in the work agreed that it could change the direction of such research. “Anyone who expects to find a single solution for problems like aging will be disappointed, because this work suggests there’s no one genetic target that could be fixed,” said Richard Lenski, an evolutionary biologist at Michigan State University (MSU).

“On the other hand, it means there are many genetic factors that can be further investigated.”

Citation Genome-wide analysis of a long-term evolution experiment with Drosophila. Molly K. Burke, Joseph P. Dunham, Parvin Shahrestani, Kevin R. Thornton, Michael R. Rose and Anthony D. Long. Nature 2010; ePub ahead of print. doi:10.1038/nature09352

Abstract

Experimental evolution systems allow the genomic study of adaptation, and so far this has been done primarily in asexual systems with small genomes, such as bacteria and yeast. Here we present whole-genome resequencing data from Drosophila melanogaster populations that have experienced over 600 generations of laboratory selection for accelerated development. Flies in these selected populations develop from egg to adult ~20% faster than flies of ancestral control populations, and have evolved a number of other correlated phenotypes. On the basis of 688,520 intermediate-frequency, high-quality single nucleotide polymorphisms, we identify several dozen genomic regions that show strong allele frequency differentiation between a pooled sample of five replicate populations selected for accelerated development and pooled controls. On the basis of resequencing data from a single replicate population with accelerated development, as well as single nucleotide polymorphism data from individual flies from each replicate population, we infer little allele frequency differentiation between replicate populations within a selection treatment. Signatures of selection are qualitatively different than what has been observed in asexual species; in our sexual populations, adaptation is not associated with ‘classic’ sweeps whereby newly arising, unconditionally advantageous mutations become fixed. More parsimonious explanations include ‘incomplete’ sweep models, in which mutations have not had enough time to fix, and ‘soft’ sweep models, in which selection acts on pre-existing, common genetic variants. We conclude that, at least for life history characters such as development time, unconditionally advantageous alleles rarely arise, are associated with small net fitness gains or cannot fix because selection coefficients change over time.

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TS-Si is dedicated to the acceptance, medical treatment, and legal protection of individuals correcting the misalignment of their brains and their anatomical sex, while supporting their transition into society as hormonally reconstituted and surgically corrected citizens.

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