Extra Credit: Drosophila Virilis and the Penelope Family

          In the species of Drosophila virilis, there is a family of retroelements that was first described in this specific species called Penelope. In Penelope integration, intact elements encode reserve transcriptase and an endonuclease of the UvrC type. What I recently found out is that Penelope plays a key role in the induction of D. virilis hybrid dysgenesis. This means that there are many unrelated families of transposable elements that are mobilized. In the 2002 published article, "Penelope retroelements from Drosophla virilis are active after transformation of Drosophila melanogaster" from PNAS.org, it reports the successful initiation of Penelope into D. melanogaster's germ line. This is done by P element-mediated transformation.. This transformation is done with three constructs.In the D. melanogaster genome, Penelope is actively transcribed only in lines that is transformed with a full-length Penelope clone. The transcript is indistinguishable  to dysgenic hybrids of D. virilis. Usually, new transposed Penelope elements contain very convoluted organization. The reproduction of Penelope copy number occurred in a few lines throughout a 24-month span following the transformation. Without the other two constructs and the increase of copy number, it is suggested that the 5' and/or 3' UTRs are needed for the succession of transposition in D. melanogaster. This is quite a feat as no retroelement from an insect has been actively transcribed and increased in numbers of copy after an interspecific transformation.
             Looking at this in more detail, the production of a syndrome of aberrant traits come from the activation of transposable elements (TEs) of certain families in Drosophila is called hybrid dygenesis. D. melanogaster contain three hybrid dyngenesis systems and they all related to the activation of three different TE families. These families are P, I, and hobo. The article claims " Additional examples have subsequently been reported in Drosophila and other Diptera. Among these is an unusually interesting hybrid dysgenesis system described in Drosophila virilis in which the Penelope TE family appears to play a pivotal role." There are also more unrelated TE families that are mobilized in dysgenic crosses. These families include Ulysses, Paris, Helena, Telemac, and Tvl.  These families are independently mobilized in D. melanogaster hybrid dygenesis of P, I, and hobo.
           The article continues to explain that Penelope elements have immensely high and complex organization within the species of D. virilis. The reverse transcriptase of Penelope is not highly related to major retroelement groups through a phylogenetic analysis. In a sequence study, it was predicted that the C-terminal domain within the Penelope polyprotein is an active endonuclease that is under analysis for Penelope integration. Although, it is related to intron-coded endonucleases and UvrC, bacterial repair endonuclease, but there is no retroelement encoding that has been described of the prediction of endonuclease.
          The study of the phylogeny shows that two families of Penelope elements are active in D. virilis. Elements that are different in organization but are closely identical in sequence are part of one subfamily. Another subfamily is consisted of defective copies that are highly branched apart. There are evidence within several lines that imply successive intrusion of Penelope  into D. virilis that can advance to genome reshuffling and speciation gross.

         When putting these ideas to test in a lab, the results were a success. The article explains how they introduced Penelope into the D. melanogaster genome. Three different constructs that contained Penelope were used in the transformation. Construct A had a full length Penelope clone. Construct B contains a full length Penelope open reading frame with missing 5' and 3' UTRs. Construct C contained full length Penelope with deleted 5' region within the open reading frame. All three constructs were transferred into a vector of P-element transformation and then was introduced to D. melanogaster embryos. The results showed progeny's eye color were pale yellow to white that darkened with age. Construct A transformants that recovered came to a total of ten, six for construct B, and another six for Construct C. Each individual fly established a line that was made homozygous for the construct afterwards. In conclusion, the 5' and/or 3' UTRs of Penelope indicate a successive transformation into the D. melanogaster genome.

The photo above shows the structure of Penelope copies integrated into D. melanogaster genome.