Modulação espectral da resposta fototática desencadeada por duas vias neurais paralelas no cérebro da abelha

Abstract

Positive phototaxis is a behavioral response of orientation towards light which is present in many animals. Considered an innate and usually stereotyped behavior, little is known about the neural processes underlying such response. The honeybee, Apis mellifera, presents strong positive phototaxis during situations such as leaving the hive, escaping, taking off from flowers and navigation. Previous studies have shown that the phototactic response of bees is activated by lights within a wide range of the spectrum between ultraviolet (UV) and yellow-green. Unlike color vision, which allows differentiating the wavelengths regardless of their intensities, the phototactic attraction of bees seems to depend mainly on light intensity, with unclear evidence for chromatic processing. While the broad spectral sensitivity of the positive phototaxis from UV to green suggests the participation of all three types of bee’s photoreceptors (S, M and L - with absorption peaks at ~350nm, ~440nm and ~535nm), the apparent lack of chromatic processing has led some authors to suggest the possibility that not all photoreceptors are involved in this behavioral response. In addition, we do not know how two parallel neural pathways originating from two types of visual organs present in bees - the compound eyes and the ocelli - participate and interact in this visuomotor response. Our aim was to fill those gaps through analysis of the phototactic trajectories triggered by monochromatic lights selective for each type of photoreceptor (S, M or L), presented in the dark at different levels of irradiance to bees submitted to the following treatments: 1- compound eyes occluded, 2- ocelli occluded, 3- no occlusion (control). The spectrum of each stimulus was precisely calibrated by spectroradiometry. The images recorded in the dark by an infrared camera were analyzed in MATLAB, and the following metrics were extracted from each trajectory performed by the bees toward the light stimulus in a circular arena: distance, time, deviation angle/frame. Distinct wavelengths, stimulating different photoreceptor channels of the bees, differently modulated the phototactic response in each experimental group. Light irradiance also influenced some motor parameters of the phototactic responses and this modulation was significantly different between different wavelengths. We also performed Y-maze experiments using monochromatic and polychromatic light stimuli in phototactic orientation tests in order to compare the type of phototactic processing in each pathway. Our results suggest that phototaxis in Apis mellifera bees presents linear and non-linear processing, with particularities in the neural processing of each visual pathway. The optical pathway presents linear and non-linear processing, with the spectral range related to the type of processing and the ocellar pathway mainly presents non-linear processing.