Literature: Why Do Bdelloid Rotifers Reproduce Only Asexually?

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Literature: Why Do Bdelloid Rotifers Reproduce Only Asexually?

Why Do Bdelloid Rotifers Reproduce Only Asexually?

Arkhipova, I.; Meselson, M. 2000. Transposable elements and ancient asexual taxa. Proceedings of the National Academy of Sciences, 97: 14473-14477.

Article Summary:
Scientists have never observed a male bdelloid rotifer in any of the more than 300 described species in this group. Although their monogonont relatives alternate seasonally between sexual reproduction and parthenogenesis, the bdelloids appear incapable of sexual reproduction.

One possible disadvantage of sexual reproduction is the accumulation of transposable elements. Transposable elements that are only mildly deleterious will spread in a population that is reproducing sexually as long as at least half of the offspring of an affected individual wind up with a copy. Scientists have assumed that transposable elements are widespread among eukaryotes, although many taxa have not previously been studied.

Irina Arkhipova and Matthew Meselson (2000) examined 46 species in 24 phyla for the presence of transposable elements in three families, the LINE-1 family, the gypsy family, and the mariner/Tc1 family. The LINE-1 and gypsy families of transposable elements are retroposons, inserting by way of an RNA intermediate and encoding reverse transcriptases (RTases). The mariner family are DNA transposons. These insert directly as DNA, and encode transposases rather than RTases. For all three families of transposable elements, sequences within a given family from widely divergent hosts are more similar than would be sequences from different families but the same hosts. Because the sequences were still highly divergent, Arkhipova and Meselson used a new specialized two-step PCR process to amplify the sequences.

Arkhipova and Meselson identified LINE-1 sequences in all 24 phyla studied. All but a few of the sequences obtained fit in known clades of LINE-1 elements. They found sequences in the gypsy family in 21 of 23 phyla studied. Again, the sequences fit within known clades of gypsy sequences. Both the LINE-1 and gypsy sequences matched previous reports for species whose genomes are well-known, e.g., Drosophila melanogaster. The LINE-1 and gypsy sequences were absent in the bdelloid rotifers, however. The monogonont rotifers as well as in acanthocephalans both exhibited the LINE-1 and gypsy sequences. (Some scientists place the acanthocephalans within the rotifers while others consider rotifers and acanthocephalans sister taxa). Arkhipova and Meselson reported sequences in the mariner family in 24 of 34 taxa studied, including all five species of bdelloid rotifers. Elsewhere, scientists have observed a a spotty taxonomic distribution for the mariner-like sequences.

1. Why did Arkhipova and Meselson choose to survey animal phyla to see the distribution of retroposons and DNA transposons?
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. Why are the comparisons of sequences from D. melanogaster, Caenorhabditis elegans, zebra fish and house mice to sequences found in GenBank important?
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3. Why did the authors construct dendrograms for each family and compare their sequences to those in GenBank?
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4. In asexual species such as bdelloid rotifers, retroposons will be transmitted chiefly vertically. Why would this result in selection against retroposons?
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5. What does the absence of retroposons in bdelloids and the presence of these elements in monogononts and acanthocephalans imply?
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6. What other eukaryote taxa would you examine in order to determine whether the pattern seen in bdelloids is unique or common among asexual organisms?
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1.	Why did Arkhipova and Meselson choose to survey animal phyla to see the distribution of retroposons and DNA transposons? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	Why are the comparisons of sequences from D. melanogaster, Caenorhabditis elegans, zebra fish and house mice to sequences found in GenBank important? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Why did the authors construct dendrograms for each family and compare their sequences to those in GenBank? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



4.	In asexual species such as bdelloid rotifers, retroposons will be transmitted chiefly vertically. Why would this result in selection against retroposons? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



5.	What does the absence of retroposons in bdelloids and the presence of these elements in monogononts and acanthocephalans imply? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



6.	What other eukaryote taxa would you examine in order to determine whether the pattern seen in bdelloids is unique or common among asexual organisms? To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 7: Activity

Literature: Why Do Bdelloid Rotifers Reproduce Only Asexually?