Genetic Recombination in Drosophila melanogaster

Abstract

In this experiment, genetic recombination in Drosophila melanogaster was investigated using the yeast GAL4/UAS transgenic expression system to "rescue" two visible recessive mutant traits: white eyes ( $w$ ) and yellow body ( $y$ ). Virgin female flies expressing both rescue markers ( $F_{1}$ ) were crossed with homozygous recessive tester males, and the phenotypic distribution of the resulting $F_{2}$ progeny was quantified to calculate the recombination frequency and deduce whether the transgene insertion sites were syntenic.

Introduction

Genetic recombination is the meiotic exchange of genetic material between homologous chromosomes via crossing over, generating novel allele combinations that underpin genetic diversity.

In this experiment, we utilized the GAL4/UAS bipartite expression system. Originally identified in Saccharomyces cerevisiae, the GAL4 transcription factor binds specifically to an Upstream Activating Sequence (UAS) to drive spatial and temporal expression of downstream transgenes.

We tracked two distinct transgenes:

$U A S - R N A i (y^{+})$ : Carries the wild type yellow allele ( $y^{+}$ ) as a selectable marker, rescuing the wild type brown body colour in a $y w$ mutant background.
$U A S - D c r - 2 (m w)$ : Carries the mini-white ( $m w$ ) construct, rescuing the wild type red/orange eye colour in a $y w$ background, while providing Dicer-2 endoribonuclease activity to potentiate siRNA processing.

The parental ( $F_{0}$ ) cross combined a fly carrying $U A S - R N A i (y^{+})$ with a fly carrying $U A S - D c r - 2 (m w)$ . Our objective was to measure the recombination frequency between these two autosomal transgene integration sites and determine whether they reside on the same chromosome (syntenic linkage) or on separate chromosomes (independent assortment).

Materials and Methods

Materials

$F_{1}$ Female stock: Heterozygous females with orange eyes and brown bodies.
Tester Males: Recessive homozygous mutant males expressing white eyes and yellow body ( $y w$ ), with Stubble bristles ( $S t u$ ).
Cornmeal feed vials supplemented with live yeast paste.
Continuous ${CO}_{2}$ anesthesia stage (flypad), fine sable brush, and stereo dissecting microscopes.

Methods

A small dab of yeast paste was placed on the surface of cornmeal feed to stimulate egg deposition. Six $F_{1}$ virgin females and three tester males were transferred to each vial and maintained at 25°C for 14 days. On day 14, newly emerged $F_{2}$ progeny were scored for eye colour and body colour combinations.

Results

Table 1: $F_{2}$ Progeny Phenotypic Distribution

Eye Colour	Body Colour	Count	Phenotypic Class
Orange	Yellow	46	Non-recombinant (Parental)
White	Brown	42	Non-recombinant (Parental)
White	Yellow	12	Recombinant
Orange	Brown	10	Recombinant
Total		110

Discussion

Case 1: Independent Assortment (Different Chromosomes)

If the two transgenes resided on separate chromosomes, independent assortment would yield equal proportions of the four phenotypic classes in a testcross:

\frac{y w}{y w}; \frac{U A S - D c r - 2 (m w)}{+}; \frac{+}{U A S - R N A i (y^{+})} \times \frac{y w}{Y}; \frac{+}{+}; \frac{+}{+}

Expected ratio: $1 : 1 : 1 : 1$ (25% for each class). However, the observed ratio was markedly skewed (41.8% : 38.2% : 10.9% : 9.1%), ruling out unlinked independent assortment.

Case 2: Genetic Linkage and Recombination (Same Chromosome)

Because parental non-recombinant classes (Orange/Yellow and White/Brown) comprise 88 out of 110 flies (80%), the transgenes reside on the same autosomal homologous chromosome pair in repulsion:

\frac{y w}{y w}; \frac{U A S - D c r - 2 (m w)}{U A S - R N A i (y^{+})}; \frac{+}{+}

Recombination between the two loci during female meiosis produced the recombinant classes (White/Yellow and Orange/Brown):

Recombination Frequency = \frac{Number of Recombinants}{Total Progeny} \times 100

Recombination Frequency = \frac{12 + 10}{110} \times 100 = \frac{22}{110} \times 100 = 20.0 %

A recombination frequency of 20.0% corresponds to a genetic map distance of 20.0 centimorgans (cM) between the $U A S - D c r - 2 (m w)$ and $U A S - R N A i (y^{+})$ transgene insertion sites.

References

Brand, A.H. and Perrimon, N. (1993). Targeted gene expression in Drosophila by the GAL4-UAS system. Development, 118(2), pp. 401–415.
Dietzl, G. et al. (2007). A genome-wide transgenic RNAi library for conditional gene inactivation in Drosophila. Nature, 448(7150), pp. 151–156.

Genetic Recombination in Drosophila melanogaster ​

Abstract ​

Introduction ​

Materials and Methods ​

Materials ​

Methods ​

Results ​

Table 1: F2 Progeny Phenotypic Distribution ​

Discussion ​

Case 1: Independent Assortment (Different Chromosomes) ​

Case 2: Genetic Linkage and Recombination (Same Chromosome) ​

References ​