The dark and nutrient poor cave environment exerts substantial pressure upon cave-dwelling animals. Perhaps as a consequence of these limited resources, cave-adapted animals from most major phyla exhibit a remarkable convergence in morphological and physiological changes related to cave life, including features that are both constructive (the lengthening of legs, fins and antennae, the appearance of novel behaviors, and elaboration of non-visual sensory systems) and regressive (the reduction or loss of vision and pigmentation) [1, 2]. Although these changes have been studied in a diverse set of cave animals, the genetic and evolutionary mechanisms responsible for them remain poorly understood.
Although natural selection is probably involved in the evolution of constructive cave adapted phenotypes , the evolutionary forces driving regressive changes are less certain. Darwin suggested that eye degeneration - one of the most conspicuous traits found in cave animals - evolved due to disuse . This idea was refined by others to implicate neutral mutation and genetic drift as a consequence of relaxed selection for vision in the cave environment [5, 6]. The neutral hypothesis was favored until recently when the results of new genetic and developmental studies supported the adaptive evolution of eye regression in cave animals. Three competing hypotheses have been proposed for the adaptive evolution of eye regression: (1) direct natural selection against eyes to conserve energy in the resource poor cave environment ; (2) indirect selection against eyes to open sufficient space for the elaboration of constructive characters [2, 8–10]; and (3) indirect selection against eyes due to the enhancement of traits that are negatively linked to optic development by antagonistic pleiotropy [11, 12]. Distinguishing among these hypotheses has been difficult since the genetic basis of eye reduction is unknown in cave-dwelling animals.
The teleost Astyanax mexicanus is an excellent model organism for studying the evolution of traits associated with cave life, including eye regression [5, 13–16]. Within the past few million years, at least five independent colonizations by two different migrational waves of eyed surface fish have established 29 geographically isolated Astyanax cavefish populations in northeastern Mexico [17–20]. After subsequent radiation underground, the founder cavefish populations became isolated in separate caves and evolved eye regression, reduced pigmentation or albinism, enhanced sensory systems and behavioral changes associated with cave life [5, 11, 12, 21–32]. Despite this isolation, Astyanax surface fish and cavefish are interfertile in the laboratory, allowing the evolution of constructive and regressive traits to be studied by genetic analysis.
Previous studies reported that eye degeneration in Astyanax is triggered by lens apoptosis and dysfunction due to expanded sonic hedgehog (shh) gene expression along the embryonic midline [25, 26]. Additionally, shh hyper expression was shown to increase jaw width and taste bud number, and to mediate the expansion of the forebrain and hypothalamus [11, 22]. In a recent study, Elipot et al. found that shh modifies the hypothalamic serotonergic network and increases foraging efficiency in cavefish by shifting behavior from fighting to foraging . These experiments support the hypothesis that eye regression in Astyanax has evolved at least in part as a result of indirect selection against eyes in favor of increased feeding efficiency through pleiotropy of the shh genes. However, recent genetic studies have discovered 8 to 12 quantitative trait loci (QTL) involved in eye reduction in Astyanax cavefish [5, 7, 33], but none that are linked to either the shhA or shhB genes, suggesting that upstream modulators of the SHH signaling system and/or other genetic factor(s) may be important in eye regression (for review, see ref. ).
In the present study, we launch an alternative approach to address the evolutionary mechanisms involved in eye degeneration. Assuming a positive relationship between visual decay and the evolution of constructive changes in other sensory systems - as has been proposed by many previous investigators [2, 8, 10, 34, 35] - we investigated the genetic basis of a constructive trait, the vibration attraction behavior (VAB), to evaluate its possible relationship to eye degeneration. VAB is the swimming of cavefish toward an oscillating object, a behavior that has evolved repeatedly in different Astyanax cavefish populations [27, 36, 37] and which may be present in Amblyopsid cavefish as well [38, 39]. VAB is mediated by an increase in the number and size of cranial superficial neuromasts (SN) in cavefish . Although VAB is usually absent in surface fish, a small proportion of those raised in the laboratory can show a weak form of VAB. In wild populations of surface fish, VAB is presumably deleterious since it may be easily detectable by predators . In contrast, VAB is adaptive in cavefish since it increases foraging in an environment devoid of light with sparse food and no macroscopic predators [27, 40].
Here we report the results of genetic analyses of VAB, SN enhancement and eye size. Using this approach, we discovered that the QTL underlying VAB, a specific class of SN located within the cavefish eye orbit (EO), and reduced eyes form two distinct clusters of overlapping QTL that together explain a significant portion of the genetic variation underlying these traits. By ablation of EO SN, we discovered that these sensory receptors contribute to VAB. Moreover, by shh overexpression, we also showed that induction of eye degeneration in surface fish did not induce VAB or increase EO SN, suggesting that these traits are part of an antagonistic system impacting eye formation that is independent of SHH signaling, and that the extra space opened by eye regression is insufficient to promote the appearance of these constructive traits. Therefore, we propose that the adaptive evolution of VAB and EO SN enhancement has contributed to eye degeneration in Astyanax cavefish.