The ability to repair genes can compensate for sex

Birds and bees all have sexual intercourse between males and females, but tiny creatures of the bdelloid rotifer class seem to be completely satisfied with their sexless life. They even experienced millions of years of evolution to form 370 different species.

David Mark Welch, Matthew Meselson and colleagues at the Marine Biological Research Laboratory (MBL) have figured out how these tiny organisms can avoid the usual disadvantages of asexual reproduction.

In the last two related articles published in the recent Proceedings of the National Academy of Sciences (PNAS), the research team came up with an interesting hypothesis: The bdelloid rotifer is capable of survival without sex because they have evolved a particularly effective mechanism to repair harmful mutations for their DNA. Mark Welch, a scientist at the Josephine Bay Paul Center (MBL), said: 'We think that in the bdelloid rotifer, the changes in the genome and the changes of the environment have been combined to create An organism can exist without mating '.

Their research results are of great medical significance because gene repair is an important factor in the treatment of cancer, inflammation, aging and other human diseases.In animals with sexual reproduction, the gene's repair ability is complete in the process of mitosis when the chromosomes pair (one from father, one from mother). Healthy genes on chromosomes act as a template to repair faulty genes on other chromosomes. Although bdelloid animals reproduce without mating, they create their own copies. So do they face harmful mutations?

Philodina roseola - a species of rotifer.The rotifers are completely satisfied with their non-sex life.(Photo: Image courtesy of Marine Biological Laboratory)

In the first article on PNAS, assistant scientist Matthew Meselson and Eugene Gladyshev (Harvard University) demonstrated the bdelloid rotifer's amazing ability to repair genes by shining ionizing radiation into them (rays gamma). This ray acts to break down DNA into many parts. Mark Welch said: 'We keep shining gamma rays on them more and more but they don't die . ' Even with five times the intensity of radiation - the level at which all other creatures have succumbed - the bdelloid continues to reproduce.

According to Mark Welch, 'these little creatures cannot evolve the ability of radiation resistance on Earth without any such ionizing radioactive source, unless they are created by ourselves'. . Scientists believe that bdelloid's DNA repair ability has evolved by adapting to an extremely dry environment.

The bdelloid live in erratic water environments such as temporary freshwater pools or on mosses. But they are capable of surviving when water is completely depleted at any stage of their life cycle. They just need to curl up, and lie dormant week after week; even over the months, years and years when there is water, they will rise again and multiply. Mark Welch and his colleagues demonstrated that drought, like ionizing radiation, broke the rotifer's DNA into many parts. Perhaps the mechanism that helped them survive the drought in their life cycle also protected them from ionizing radiation.

Mark Welch said: 'That is the next issue that we are investigating. How can bdelloids heal their two-chain fragment genes? Do they have better enzymes or do they have more enzymes? '

The factor that bestowed on the excellent gene repair capacity of bdelloid was described in the team's second article on PNAS. In it they gave evidence that bdelloid rotifer - like most other species - has two copies of each chromosome. But at some point in their evolution, they underwent a 'whole genome duplication' process that made each chromosome have four copies and each gene . Usually, species with the entire genome duplication phenomenon will gradually lose their transcription genes over time. But bdelloids retain most of these copies throughout their evolutionary history.

Mark Welch said: 'We believe that bdelloids have retained most of their copy genes because they are acting as prototypes to repair fault genes . ' One consequence of this gene repair is genetic modification. The gene to be repaired will eventually have the same sequence as the molded genome. Thanks to this phenomenon, it is possible that the changes took place in the genome of those who had mated. For example, a gene that codes for brown eyes corrects the gene that encodes blue eyes on the same chromosome; as a result, it turned the blue-eye gene into a brown-eyed gene.

Mark Welch concludes: 'We think that genetic modification due to the ability to repair genes created during the adaptation to drought has provided enough advantages for bdelloid to exist.'

Refer:
+ The first reference article: Gladyshev, E., and M. Meselson. 2008; The incredible resistance of Bdelloid rotifer to ionizing radiation - PNAS 105 (13): 5139-5144.
+ Second reference article: Mark Welch, DB, JL Mark Welch and M. Meselson.2008;Evidence of degenerative tetraploid form in Bdelloid rotifer - PNAS 105 (13): 5145-5149.