Fruitless Gene

There is a gene in Drosophila melanogaster that is a multifunctional gene called fruitless or fru gene. It comprises of sex-specific function within regulation of sexual behavior and sex-nonspecific function in males, affecting adult growth and external morphology. Focusing on function, a normal fruitless gene is required for proper development for a variety of anatomical structure that is quite necessary for courtship. This gene is a pure example of how a gene can regulate the function and development of neurons that are tied to innate behavior.

the fru gene takes part within Drosophila melanogaster as a representative of Drosophila sex determination regulatory hierarchy. this gene's multiple functions are encoded by a set of sex-specific and sex-nonspecific transcripts that are generated by four promoters and alternative splicing at the 5' end and 3' end of the transcripts, according to the article "Molecular genetic dissection of the sex-specific and vital functions of the Drosophila melanogaster sex-determination gene fruitless" from NCBI.nih.gov. This article continues to explain that the fru's promoter transcripts have a fairly long open reading frames (ORF) that code for a relative protein from the BTB-Zn family. Transcripts are spliced sex-specifically from the promoter under the female's control of transformer (TRA) and transformer-2 (TRA-2) proteins. Males is quite different. Males has a default splice. These transcripts are expressed in small neurons of the central nervous system and is known to be the reason behind a fru male's sexual behavior. For females, the article goes on to say, "the splicing of the PI-derived transcripts give rise to mRNAs with the potential to encode proteins with a BTB domain at their amino termini and one of three alternative Zn finger pairs at their carboxy termini." For the male's default splicing, the PI transcripts generate male-specific mRNAs that encode proteins that differ to the predicted proteins that females make. A Pubmed.gov article titled "Prediction of protein function from protein sequence and structure" shares that three promoters P2, P3, and P4 encode sex-nonspecific proteins that differ from proteins of male-specific due to having a short strand of amino acids. the roles of these sex-nonspecific proteins are unknown, but it has been said that possibly one or more of those proteins are a vital function for the fru gene.

The function of the fru gene is the most interesting aspect, in my opinion. According to the article, "Brain sex differences and function of the fruitless gene in Drosophila" from Ebscohost.com, the genetic characterization that was demonstrated of the gene is that the bisexual phenotype of the fru mutants has correlation with two breakpoints of inversion within the chromosome, including one that suggests inadequate sex appeal on the male fly which causes his to be courted by other males. The alleles from the fru gene contain defect in the development of a male-specific muscle names the muscle of Lawrence (MOL). this is a pair of large muscles that run along the tergite within the fifth abdominal segment of adult males. MOL is useful as if characterizing the fru gene's function. Undergrowth of the MOL was shown in males with original fru1 allele and other male's alleles are vacant or lacking the MOL, which gives different levels of functions. It was shown in studies that MOL forms when the sex of innervating motoneurons is male and MOL defects in males with fru alleles arose from a deficiency in the masculinization of innervating motoneurons. There are also suggestions that there are unknown classifications in sex-determination within tra and tra-2. The article, "Control of Male Sexual Behavior and Sexual Orientation in Drosophila by the fruitless Gene" published on Sciencedirect.com claims "Constitutive expression of the male form of the Dsx protein transforms females into morphologically wild-type males, but they do not court. These results suggest that there is a previously unrecognized branch (or branches) in the sex-determination hierarchy just downstream of traand tra-2 that controls MOL development and many aspects of male sexual behavior. Interestingly, mosaic analysis suggests that the development of the MOL depends on the sex of the neurons that innervate it."  They also explain that the nervous system function depend on both sexual and MOL development, and the new sex determination hierarchy branch operates there. It also has been said that the alteration of sexual orientation is tied with feminizing certain brain regions and a misexpression of the white gene within the brain. We know that many genes can affect the courtship performance within Drosophila males but fru is a unique gene that seems to be specific to male courtship. Singing through copulation seems to be missing or abnormal in fru mutant males and they become sterile if they fail to copulate. fru males also do not discriminate between female or male when it comes to courtship. they form a chain which each male is both courting and being court. With all these phenotypic effect within mutant males, there is no effect of fru with females.

Apart from phenotypic effects, MOL may show an ancestral state of development in lineages that outrank the radiation of subfamily Drosophilinae. The article "Functional Conservation of the fruitless Male Sex-Determination Gene Across 250 Myr of Insect Evolution" from Oxfordjournals.com, they explain that selective constraints on fru gene are grand which gives us a narrowly conserved gene. This is clearly shown in doublesex (dsx). The article explains, "This is certainly the case for doublesex (dsx), a terminal effector gene of the Drosophila sex-determination pathway responsible largely as a switch gene for the developmental pathways that give rise to male versus female anatomical features and that shows conservation among nematodes, rodents, and humans." It is at an understanding that this gene has been maintained from the malaria mosquito, Anopheles gambiae and it has been concluded that the fru gene is an ancestral gene among dipteran insects. There is also evidence of phenotypic resemblance of the male mosquito, arguing the fru-like muscle is also conserved. All these functions, specifications, and behavior form a strong explanation of fru and it's evolution.

Bibliography

Anand, A. Molecular Genetic Dissection of the Sex-Specific and Vital Functions of the Drosophila melanogaster Sex Determination Gene Fruitless .  http://www.ncbi.nlm.nih.gov/pmc/?cmd=Search&term=00166731%5Bjour%5D+AND+158%5Bvolume%5D+AND+1569%5Bpage%5D+AND+2001%5Bpdat%5D+AND+Anand%5Bauth%5D

Gailey, D. A. Functional Conservation of the fruitless Male Sex-Determination Gene Across 250 Myr of Insect Evolution. Molecular Biology and Evolution. http://mbe.oxfordjournals.org/content/23/3/633.full

Lesk, A. M., & Whisstock, J. C. Prediction of protein function from protein sequence and structure. Quarterly Reviews of Biophysics, 307-340. http://search.proquest.com.ezproxy.nu.edu/docview/212035231/fulltextPDF?accountid=25320

Ryner, L. Control of Male Sexual Behavior and Sexual Orientation in Drosophila by the fruitless Gene. Cell, 1079-1089. http://www.sciencedirect.com/science/article/pii/S0092867400818024

Yamamoto, D. Brain Sex Differences and Function of the fruitless Gene in Drosophila. http://web.b.ebscohost.com.ezproxy.nu.edu/ehost/pdfviewer/pdfviewer?sid=7fcf93e1-ffc1-4133-a91e-2f66c69ea72a%40sessionmgr110&vid=2&hid=126

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.