Influence of the White locus on the courtship behavior of Drosophila males

Abstract

Since its discovery by Morgan, the Drosophila white gene has become one of the most intensely studied genes and has been widely used as a genetic marker. Earlier reports that over- and misexpression of White protein in Drosophila males leads to male-male courtship implicated white in courtship control. While previous studies suggested that it is the mislocalization of White protein within cells that causes the courtship phenotype, we demonstrate here that also the lack of extra-retinal White can cause very similar behavioral changes. Moreover, we provide evidence that the lack of White function increases the sexual arousal of males in general, of which the enhanced male-male courtship might be an indirect effect. We further show that white mutant flies are not only optomotor blind but also dazzled by the over-flow of light in daylight. Implications of these findings for the proper interpretation of behavioral studies with white mutant flies are discussed.

Figure 1. Role of White protein in the biosynthesis of Drosophila eye pigments and the neurotransmitters serotonin and dopamine.

The White protein is an ABC transporter that, in combination with the Scarlet protein, transports tryptophan and, in combination with the Brown protein, guanine across the cell membrane into the cytoplasm . Tryptophan is a precursor of the Drosophila Ommochrome pigment xanthommatin (brown) , but is also a precursor of the neurotransmitter serotonin , as illustrated on the left. Guanine is a precursor of tetrahydrobiopterin (BH₄), which in turn is a precursor of most Drosopterins (red eye pigments of Drosophila) but also an essential cofactor in the conversion of tyrosine to dopamine, as indicated on the right, and of tryptophan to serotonin, as depicted on the left , .

Figure 2. w¹¹¹⁸ flies are not only optomotor blind but also dazzled by light.

Courtship vigor indices were measured in single-choice courtship assays with mature males of indicated genotypes and receptive (A) or decapitated (B) Ore-R virgin females in daylight (yellow columns), under dim red light (red columns), or under low-intensity light conditions (orange columns). The numbers below the columns indicate the number of couples observed that initiated courtship, and error bars always represent double standard errors of the mean. Data for ninaB^360d males in panel A are from .

Figure 3. Lack of extra-retinal White function influences sexual orientation of males.

Courtship vigor indices were measured in single-choice courtship assays with mature males of indicated genotypes and decapitated males (hatched columns) or decapitated females (filled columns) in the dark (red columns) or in daylight (yellow columns). The numbers below the columns indicate the number of males observed that initiated courtship. Data for Ore-R males are from .

Figure 4. Chaining behavior of males is stimulated by a w¹¹¹⁸ background.

(A) In addition to the average chaining indices for groups of eight males of indicated genotypes, the number of groups for which chaining was observed over the total number of groups tested is shown. Error bars represent double standard errors. The result for Poxn-pRes males is from . (B) Courtship chain of eight w¹¹¹⁸; Poxn-pRes males. The picture was taken 10 min after eight mature, but sexually naïve, w¹¹¹⁸; Poxn-pRes males were placed together into a small Petri dish.

Figure 5. Increased sexual arousal of w¹¹¹⁸ males.

(A–C) Male courtship parameters, the courtship vigor index (A), the fraction of males initiating courtship (B), and the average latency (in seconds) till courtship initiation (C), were measured in single-choice courtship assays with mature males of indicated genotypes and decapitated virgins (filled columns) or males (hatched columns) in the dark. (D) Courtship vigor indices of Poxn-pRes; Or83b² males, courting intact or decapitated females, were measured in single-choice courtship assays in daylight. Poxn-pRes; Or83b² males were either in a w¹¹¹⁸ or w ⁺ background. It should be noted that w¹¹¹⁸; Poxn-pRes; Or83b² males carry two transgenes both of which include a mini-white marker gene (see Materials and Methods) that fully rescues pigmentation. Results for Ore-R and Poxn-pRes males in (A–C) and for Poxn-pRes; Or83b² males in (D) are from .

Figure 6. Effect of white mutation on sexual arousal of males.

Models explaining possible effects of extra-retinal lack of White function on general sexual arousal of males. Increased sexual arousal of white males over time t may be caused by (A) a change in the general alertness from a ground state in wild-type flies (solid blue line) to an enhanced state of alertness in w¹¹¹⁸ mutants (broken blue line), resulting in a shortened, sensitized period of latency to sustained courtship, t_s, as compared to the control period of latency, t_c, or (B) a reduction of the threshold from its wild-type (solid red line) to a reduced w¹¹¹⁸ level (broken red line), which relevant stimuli need to reach to elicit sustained courtship, also resulting in a shortened, sensitized period of latency, t_s. Since the processing of various sensory information by the CNS is not affected in either model, both changes can be taken to result from an enhanced sensitivity of the CNS to courtship stimuli. (C) Alternatively, w¹¹¹⁸ males may reach the threshold level for sustained courtship faster (broken green line in w¹¹¹⁸ mutants with slope greater than that of solid green line in wild-type flies) because of an altered processing or integration of courtship information by the CNS or an enhanced sensitivity to relevant stimuli, for example pheromones, of sensory neurons in the PNS. Also in this model, the augmented sexual arousal leads to a shortened, sensitized period of latency to sustained courtship, t_s, because of a steeper slope of the broken versus the solid green line.