The Discovery of XY Sex Chromosomes in a \u3cem\u3eBoa\u3c/em\u3e and \u3cem\u3ePython\u3c/em\u3e

For over 50 years, biologists have accepted that all extant snakes share the same ZW sex chromosomes derived from a common ancestor [1, 2, 3], with different species exhibiting sex chromosomes at varying stages of differentiation. Accordingly, snakes have been a well-studied model for sex chromosome evolution in animals [1, 4]. A review of the literature, however, reveals no compelling support that boas and pythons possess ZW sex chromosomes [2, 5]. Furthermore, phylogenetic patterns of facultative parthenogenesis in snakes and a sex-linked color mutation in the ball python (Python regius) are best explained by boas and pythons possessing an XY sex chromosome system [6, 7]. Here we demonstrate that a boa (Boa imperator) and python (Python bivittatus) indeed possess XY sex chromosomes, based on the discovery of male-specific genetic markers in both species. We use these markers, along with transcriptomic and genomic data, to identify distinct sex chromosomes in boas and pythons, demonstrating that XY systems evolved independently in each lineage. This discovery highlights the dynamic evolution of vertebrate sex chromosomes and further enhances the value of snakes as a model for studying sex chromosome evolution

Consecutive Virgin Births in the New World Boid Snake, the Colombian Rainbow Boa, Epicrates maurus

Until recently, facultative automictic parthenogenesis within the squamate reptiles exhibiting ZZ:ZW genetic sex determination has resulted in single reproductive events producing male (ZZ) or female (ZW) offspring. With the recent discovery of viable parthenogenetically produced female (WW) Boa constrictors, the existence of further parthenogenetic events resulting in WW females was questioned. Here, we provide genetic evidence for consecutive virgin births by a female Colombian rainbow boa (Epicrates maurus), resulting in the production of WW females likely through terminal fusion automixis. Samples were screened at 22 microsatellite loci with 12 amplifying unambiguous products. Of these, maternal heterozygosity was observed in 4, with the offspring differentially homozygous at each locus. This study documents the first record of parthenogenesis within the genus Epicrates, a second within the serpent lineage Boidae, and the third genetically confirmed case of consecutive virgin births of viable offspring within any vertebrate lineage. Unlike the recent record in Boa constrictors, the female described here was isolated from conspecifics from birth, demonstrating that males are not required to stimulate parthenogenetic reproduction in this species and possibly other Boa

Maturation of GABAergic Inhibition Promotes Strengthening of Temporally Coherent Inputs among Convergent Pathways

Spike-timing-dependent plasticity (STDP), a form of Hebbian plasticity, is inherently stabilizing. Whether and how GABAergic inhibition influences STDP is not well understood. Using a model neuron driven by converging inputs modifiable by STDP, we determined that a sufficient level of inhibition was critical to ensure that temporal coherence (correlation among presynaptic spike times) of synaptic inputs, rather than initial strength or number of inputs within a pathway, controlled postsynaptic spike timing. Inhibition exerted this effect by preferentially reducing synaptic efficacy, the ability of inputs to evoke postsynaptic action potentials, of the less coherent inputs. In visual cortical slices, inhibition potently reduced synaptic efficacy at ages during but not before the critical period of ocular dominance (OD) plasticity. Whole-cell recordings revealed that the amplitude of unitary IPSCs from parvalbumin positive (Pv+) interneurons to pyramidal neurons increased during the critical period, while the synaptic decay time-constant decreased. In addition, intrinsic properties of Pv+ interneurons matured, resulting in an increase in instantaneous firing rate. Our results suggest that maturation of inhibition in visual cortex ensures that the temporally coherent inputs (e.g. those from the open eye during monocular deprivation) control postsynaptic spike times of binocular neurons, a prerequisite for Hebbian mechanisms to induce OD plasticity

The emerging phylogenetic pattern of parthenogenesis in snakes

Parthenogenesis occurs across a variety of vertebrate taxa. Within squamate reptiles (lizards and snakes), a group for which the largest number of cases has been documented, both obligate and facultative types of parthenogenesis exists, although the obligate form in snakes appears to be restricted to a single basal species of blind snake, Indotyphlops braminus. By contrast, a number of snake species that otherwise reproduce sexually have been found capable of facultative parthenogenesis. Because the original documentation of this phenomenon was restricted to subjects held in captivity and isolated from males, facultative parthenogenesis was attributed as a captive syndrome. However, its recent discovery in nature shifts the paradigm and identifies this form of reproduction as a potentially important feature of vertebrate evolution. In light of the growing number of documented cases of parthenogenesis, it is now possible to review the phylogenetic distribution in snakes and thus identify subtle variations and commonalities that may exist through the characterization of its emerging properties. Based on our findings, we propose partitioning facultative parthenogenesis in snakes into two categories, type A and type B, based on the sex of the progeny produced, their viability, sex chromosome morphology, and ploidy, as well as their phylogenetic position. Furthermore, we introduce a hypothesis (directionality of heterogamety hypothesis) to explain the production of female-only parthenogens in basal alethinophidian snakes and male-only parthenogens in caenophidian (advanced) snakes

Crotalus scutulatus (Mohave Rattlesnake). Reproduction and Sexual Receptivity

This video demonstrates behavior. Most videos in this collection have no audible language and for those that do, the language isn't necessary to understand the behavior. For that reason, transcripts are not provided.Integrative Biolog

Data from: Seed ingestion and germination in rattlesnakes: overlooked agents of rescue and secondary dispersal

Seed dispersal is a key evolutionary process and a central theme in the population ecology of terrestrial plants. The primary producers of most land-based ecosystems are propagated by and maintained through various mechanisms of seed dispersal that involve both abiotic and biotic modes of transportation. By far the most common biotic seed transport mechanism is zoochory, whereby seeds, or fruits containing them, are dispersed through the activities of animals. Rodents are one group of mammals that commonly prey on seeds (granivores) and play a critical, often destructive, role in primary dispersal and the dynamics of plant communities. In North America, geomyid, heteromyid and some sciurid rodents have specialized cheek pouches for transporting seeds from plant source to larder, where they are often eliminated from the pool of plant propagules by consumption. These seed-laden rodents are commonly consumed by snakes as they forage, but unlike raptors, coyotes, bobcats, and other endothermic predators which eat rodents and are known or implicated to be secondary seed dispersers, the role of snakes in seed dispersal remains unexplored. Here, using museum-preserved specimens, we show that in nature three desert-dwelling rattlesnake species consumed heteromyids with seeds in their cheek pouches. By examining the entire gut we discovered, furthermore, that secondarily ingested seeds can germinate in rattlesnake colons. In terms of secondary dispersal, rattlesnakes are best described as diplochorous. Because seed rescue and secondary dispersal in snakes has yet to be investigated, and because numerous other snake species consume granivorous and frugivorous birds and mammals, our observations offer direction for further empirical studies of this unusual but potentially important channel for seed dispersal

Exceptional long-term sperm storage by a female vertebrate.

Females of many vertebrate species have the capacity to store sperm within their reproductive tracts for prolonged periods of time. Termed long-term sperm storage, this phenomenon has many important physiological, ecological, and evolutionary implications, particularly to the study of mating systems, including male reproductive success and post-copulatory sexual selection. Reptiles appear particularly predisposed to long-term sperm storage, with records in most major lineages, with a strong emphasis on turtles and squamates (lizards, snakes, but not the amphisbaenians). Because facultative parthenogenesis is a competing hypothesis to explain the production of offspring after prolonged separation from males, the identification of paternal alleles through genetic analysis is essential. However, few studies in snakes have undertaken this. Here, we report on a wild-collected female Western Diamond-backed Rattlesnake, Crotalus atrox, maintained in isolation from the time of capture in September 1999, that produced two healthy litters approximately one and six years post capture. Genetic analysis of the 2005 litter, identified paternal contribution in all offspring, thus rejecting facultative parthenogenesis. We conclude that the duration of long-term sperm storage was approximately 6 years (71 months), making this the longest period over which a female vertebrate has been shown to store sperm that resulted in the production of healthy offspring

Crotalus atrox (Western Diamond-backed Rattlesnake). Courtship and Phenology

This video demonstrates behavior. Most videos in this collection have no audible language and for those that do, the language isn't necessary to understand the behavior. For that reason, transcripts are not provided.Integrative Biolog