Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Ashley Bear, Kathleen L. Prudic, Antónia Monteiro, William J. Etges.
It is well established that steroid hormones regulate sexual behavior in vertebrates via organizational and activational effects. However, whether the organizational/activational paradigm applies more broadly to the sexual behavior of other animals such as insects is not well established. Here we describe the hormonal regulation of a sexual behavior in the seasonally polyphenic butterfly Bicyclus anynana is consistent with the characteristics of an organizational effect. By measuring hormone titer levels, quantifying hormone receptor gene expression in the brain, and performing hormone manipulations, we demonstrate steroid hormone signaling early in pupal development has a latent effect on adult male sexual behavior in B. anynana. These findings suggest the organizational/activational paradigm may be more highly conserved across animal taxa than previously thought.
Hormones play a major role in regulating many sexual behaviors in vertebrates, but it is only recently that studies have demonstrated hormonal regulation of sexual behavior in insects [1–13]. In the vertebrate literature, hormonal effects on the parts of the brain that regulate sexual behavior (e.g., the neural substrates of sexual behavior) have often been defined as either organizational or activational. Organizational effects are traditionally defined as permanent modifications to the brain that occur during a critical period early in development and act to define certain characteristics of adult sexual behavior. In contrast, activational effects are defined as reversible effects of hormones on the brain which modify adult behavior by stimulating the neural substrates previously defined by organizational effects [1, 4, 6, 14]. Although it is now recognized that these definitions often represent extremes on a continuum of how hormones can affect sexual behavior, this paradigm remains prevalent in the literature as a powerful conceptual framework for describing how hormones regulate development [4, 6].
Here we demonstrate that variation in hormone signaling during B. anynana development influences plasticity in an adult behavioral trait. By examining hormone titers, hormone receptor expression, and performing functional endocrine manipulations during the pupal stage of development, which was previously shown to be the beginning of the critical period for the courtship rate plasticity , we show that differences in hormone titers play a role in regulating the courtship rate plasticity in adult male B. anynana. We first found significant differences in 20E hormone titers between DS and WS males at 14%, 30%, and 50% of pupal development (Fig 1). At each of these developmental periods, WS pupae had higher titers than DS pupae. However, WS and DS males had equivalent EcR expression levels in the brain throughout the pupal stages investigated, suggesting no differences in sensitivity to 20E (Fig 2). Injections of 20E into DS male pupa, but not injections of vehicle, significantly elevated 20E titers post injection (Fig 4). Finally, injection of DS males with 20E at 30% of pupal development, but not at 50%, was sufficient to increase the courtship rate of DS butterflies (Fig 3B). Thus, the higher 20E titer at 30% of pupal development in WS males plays an important regulatory role in the adult courtship rate plasticity. We found no evidence to support the idea that the higher 20E titer at 50% of pupal development influences male courtship.
Our results demonstrate that a steroid hormone acting during development can have a latent effect on adult sexual behavior in an insect. These findings are consistent with the characteristics of an organizational effect, a mechanism of hormone action that has rarely been documented in the context of insect sexual behavior. These findings raise questions about whether the organizational/activation paradigm of hormonal regulation of sexual behavior applies more broadly across the animal tree of life than previously thought.