Genital evolution: cock-a-doodle-don't

Loss of Avian Intromittent Organs as a Sperm Competition Strategy: A Race to Be Last

AbstractThe loss of the intromittent organ (IO) in the majority of birds remains unexplained. Here, I propose that IO loss results from sperm competition in the context of the unique avian sperm storage system. The first stage of fertilization is the movement of sperm through the vagina from the site of ejaculation to the sperm storage tubules (SSTs) at the uterovaginal junction. In a second stage, sperm are released from the SSTs and move through the uterus to the site of fertilization in the infundibulum. Last-male fertilization precedence can occur if sperm that arrive later at the uterovaginal junction occupy uterus-side SSTs, which then have a head start in the race to the infundibulum with each ovulation. Under such "last-in, first-out" conditions, there is strong sperm competition to arrive later at the SSTs. Consequently, the optimal male strategy would be placement of the ejaculate at the cloacal opening to allow any other sperm in the vagina to reach the SSTs first. Cloacal placement is effectively achieved by loss of the IO. The evolution of altricial development in Neoaves, the largest clade that lacks IOs, created conditions that favor IO loss. Specifically, the smaller clutch sizes and hatching asynchrony of altricial birds increase the intensity of sperm competition for fertilization of early eggs in the laying sequence and thus the selective advantage of later arrival at the SSTs. The rarity of IO loss among all animals suggests that the complex mechanism of avian fertilization creates unique conditions for sperm competition.

β-Defensins: Farming the Microbiome for Homeostasis and Health

Diverse commensal populations are now regarded as key to physiological homeostasis and protection against disease. Although bacteria are the most abundant component of microbiomes, and the most intensively studied, the microbiome also consists of viral, fungal, archael, and protozoan communities, about which comparatively little is known. Host-defense peptides (HDPs), originally described as antimicrobial, now have renewed significance as curators of the pervasive microbial loads required to maintain homeostasis and manage microbiome diversity. Harnessing HDP biology to transition away from non-selective, antibiotic-mediated treatments for clearance of microbes is a new paradigm, particularly in veterinary medicine. One family of evolutionarily conserved HDPs, β-defensins which are produced in diverse combinations by epithelial and immune cell populations, are multifunctional cationic peptides which manage the cross-talk between host and microbes and maintain a healthy yet dynamic equilibrium across mucosal systems. They are therefore key gatekeepers to the oral, respiratory, reproductive and enteric tissues, preventing pathogen-associated inflammation and disease and maintaining physiological normality. Expansions in the number of genes encoding these natural antibiotics have been described in the genomes of some species, the functional significance of which has only recently being appreciated. β-defensin expression has been documented pre-birth and disruptions in their regulation may play a role in maladaptive neonatal immune programming, thereby contributing to subsequent disease susceptibility. Here we review recent evidence supporting a critical role for β-defensins as farmers of the pervasive and complex prokaryotic ecosystems that occupy all body surfaces and cavities. We also share some new perspectives on the role of β-defensins as sensors of homeostasis and the immune vanguard particularly at sites of immunological privilege where inflammation is attenuated.

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

Genital coevolution between the sexes is expected to be common because of the direct interaction between male and female genitalia during copulation. Here we review the diverse mechanisms of genital coevolution that include natural selection, female mate choice, male-male competition, and how their interactions generate sexual conflict that can lead to sexually antagonistic coevolution. Natural selection on genital morphology will result in size coevolution to allow for copulation to be mechanically possible, even as other features of genitalia may reflect the action of other mechanisms of selection. Genital coevolution is explicitly predicted by at least three mechanisms of genital evolution: lock and key to prevent hybridization, female choice, and sexual conflict. Although some good examples exist in support of each of these mechanisms, more data on quantitative female genital variation and studies of functional morphology during copulation are needed to understand more general patterns. A combination of different approaches is required to continue to advance our understanding of genital coevolution. Knowledge of the ecology and behavior of the studied species combined with functional morphology, quantitative morphological tools, experimental manipulation, and experimental evolution have been provided in the best-studied species, all of which are invertebrates. Therefore, attention to vertebrates in any of these areas is badly needed.

Endocrine regulation and sexual differentiation of avian copulatory sexually selected characters

Reproductive specializations in birds have provided intriguing model systems to better understand the role of endocrine mechanisms that regulate phenotype expression and the action of sexual selection. A comparative approach can elucidate how endocrine systems associated with control of sexual differentiation, sexual maturation, and reproductive physiology and behavior have diversified. Here we compare the copulatory sexually selected traits of two members of the galloanseriform superfamily: quail and ducks. Japanese quail have a non-intromittent penis, and they have evolved a unique foam gland that is known to be involved in post-copulatory sexual selection. In contrast, ducks have maintained a large intromittent penis that has evolved via copulatory male-male competition and has been elaborated in a sexually antagonistic race due to sexual conflict with females over mating. These adaptations function in concert with sex-specific and, in part, species-specific behaviors. Although the approaches to study these traits have been different, exploring the differences in neuroendocrine regulation of sexual behavior, development and seasonality of the foam gland and the penis side by side, allow us to suggest some areas where future research would be productive to better understand the evolution of novelty in sexually selected traits.