The bachelorette: Female Siamese fighting fish avoid males exposed to an estrogen mimic

Independent and synergistic effects of microplastics and endocrine-disrupting chemicals on the reproductive social behavior of fathead minnows (Pimephales promelas)

Microplastics (MPs) have become an environmental concern in recent years, with most research focused on the physiological effects of exposure. Comparatively little consideration has been given to the potential behavioral impacts of exposure, which may also have fitness consequences for individuals. Moreover, MPs can serve as vectors for endocrine-disrupting chemicals and other locally co-occurring contaminants known to impair behavioral responses. This project aimed to determine whether MPs alone or in association with a common environmental EDC (17-alpha ethinyl estradiol; EE2) alter reproductive behavior and decision-making in fish. Male and female fathead minnows (Pimephales promelas) were exposed to MPs associated with either a low (10 ng/L; MPEE2 10) or high (50 ng/L, MPEE2 50) concentration of EE2, or MPs without EE2 (MPvirgin) for 30 days via a dietary feeding protocol. Behavioral trials were conducted on Day 31 to determine the effects of exposure on male-female social interactions. The expression of male sexually selected traits, including courtship, was unaffected by exposure. However, non-exposed females in all treatment groups trended toward discrimination against exposed males, which reached statistical significance for the MPEE2 50 group. Female fish exposed to MPs, alone or in association with EE2, were equally likely to approach and associate with non-exposed and exposed males. The results from this study suggest that MPs may alter social behavior in fishes and that the behavioral impacts of exposure may be more strongly pronounced in females than males. Such individual-level changes in fitness have the potential to impact population size, with downstream effects on the broader aquatic community.

Behavioral and endocrine responses to noninteractive live and video conspecifics in males of the Siamese fighting fish

The physiological mechanisms underlying variation in aggression in fish remain poorly understood. One possibly confounding variable is the lack of standardization in the type of stimuli used to elicit aggression. The presentation of controlled stimuli in videos, a.k.a. video playback, can provide better control of the fight components. However, this technique has produced conflicting results in animal behavior studies and needs to be carefully validated. For this, a similar response to the video and an equivalent live stimulus needs to be demonstrated. Further, different physiological responses may be triggered by live and video stimuli, and it is important to demonstrate that video images elicit appropriate physiological reactions. Here, the behavioral and endocrine responses of male Siamese fighting fish Betta splendens to a matched-for-size conspecific fighting behind a one-way mirror, presented live or through video playback, were compared. The video playback and live stimulus elicited a strong and similar aggressive response by the focal fish, with a fight structure that started with stereotypical threat displays and progressed to overt attacks. Postfight plasma levels of the androgen 11-ketotestosterone were elevated as compared to controls, regardless of the type of stimuli. Cortisol also increased in response to the video images, as previously described for live fights in this species. These results show that the interactive component of a fight and its resolution are not needed to trigger an endocrine response to aggression in this species. The study also demonstrates for the first time in a fish a robust endocrine response to video stimuli and supports the use of this technique for researching aggressive behavior in B. splendens.

Evolutionary novelty in communication between the sexes

The diversity of signalling traits within and across taxa is vast and striking, prompting us to consider how novelty evolves in the context of animal communication. Sexual selection contributes to diversification, and here we endeavour to understand the initial conditions that facilitate the maintenance or elimination of new sexual signals and receiver features. New sender and receiver variants can occur through mutation, plasticity, hybridization and cultural innovation, and the initial conditions of the sender, the receiver and the environment then dictate whether a novel cue becomes a signal. New features may arise in the sender, the receiver or both simultaneously. We contend that it may be easier than assumed to evolve new sexual signals because sexual signals may be arbitrary, sexual conflict is common and receivers are capable of perceiving much more of the world than just existing sexual signals. Additionally, changes in the signalling environment can approximate both signal and receiver changes through a change in transmission characteristics of a given environment or the use of new environments. The Anthropocene has led to wide-scale disruption of the environment and may thus generate opportunity to directly observe the evolution of new signals to address questions that are beyond the reach of phylogenetic approaches.

Improving the Chromosome-Level Genome Assembly of the Siamese Fighting Fish (Betta splendens) in a University Master's Course

Ever decreasing costs along with advances in sequencing and library preparation technologies enable even small research groups to generate chromosome-level assemblies today. Here we report the generation of an improved chromosome-level assembly for the Siamese fighting fish (Betta splendens) that was carried out during a practical university master’s course. The Siamese fighting fish is a popular aquarium fish and an emerging model species for research on aggressive behavior. We updated the current genome assembly by generating a new long-read nanopore-based assembly with subsequent scaffolding to chromosome-level using previously published Hi-C data. The use of ∼35x nanopore-based long-read data sequenced on a MinION platform (Oxford Nanopore Technologies) allowed us to generate a baseline assembly of only 1,276 contigs with a contig N50 of 2.1 Mbp, and a total length of 441 Mbp. Scaffolding using the Hi-C data resulted in 109 scaffolds with a scaffold N50 of 20.7 Mbp. More than 99% of the assembly is comprised in 21 scaffolds. The assembly showed the presence of 96.1% complete BUSCO genes from the Actinopterygii dataset indicating a high quality of the assembly. We present an improved full chromosome-level assembly of the Siamese fighting fish generated during a university master’s course. The use of ∼35× long-read nanopore data drastically improved the baseline assembly in terms of continuity. We show that relatively in-expensive high-throughput sequencing technologies such as the long-read MinION sequencing platform can be used in educational settings allowing the students to gain practical skills in modern genomics and generate high quality results that benefit downstream research projects.

Mating under the influence: male Siamese fighting fish prefer EE2-exposed females

Countless pharmaceuticals and endocrine disrupting chemicals (EDCs) exist on the market with more added each day. Many of these compounds are not removed during the wastewater treatment process and enter bodies of water in their active form. EDCs are known to have physiological and behavioral effects in exposed organisms. Exposure to the synthetic estrogen 17α-ethinylestradiol (EE2), a common EDC found in birth control pills, has been found to lead to population collapse after only a few generations in some fish species. Mechanisms identified as potential driving forces for collapse include feminization of males and altered fecundity in both sexes. However, an additional way in which EE2 could lead to population collapse is by altering courtship behavior, which could then change mating preferences and decrease mating opportunities. The current study had the following objectives: determine if exposing female Siamese fighting fish, Betta splendens, to EE2 changes mate choice in males; assess if the dose and duration of female exposure matters; and examine if exposing males to EE2 influences their mating preferences. Both unexposed and exposed males were presented with pairs of females that differed in EE2 dose and exposure duration. The results indicate that males were more responsive to EE2-exposed females than unexposed females, with males being most responsive to females exposed to the low versus high dose. Furthermore, exposed males responded less overall than unexposed males. If EE2 concentration increases in the environment, the likelihood of successful mating could decrease and, therefore, potentially lead to adverse population impacts.

Behavioral consequences of dietary exposure to crude oil extracts in the Siamese fighting fish (Betta splendens)

Uptake by fishes of crude oil and its polycyclic aromatic hydrocarbons (PAHs) components occurs via gills, dietary intake, or diffusion through the skin. Dietary exposure to crude oil and its components is environmentally relevant, and induces physiological and morphological disruptions in fish. However, the impacts of crude oil on fish social and reproductive behaviors and thus the possible influences on reproductive success are poorly understood. As a part of their intraspecific interactions, male Siamese fighting fish (Betta splendens) exhibit highly stereotypic behavioral and territorial displays. This makes this species a tractable model for testing crude oil effects on behavior. After 2 weeks of acclimation at 29 °C, male adult betta fish were divided into three groups and fed for 4 weeks with food spiked with water (control), low oil concentrations or high oil concentrations (∑Total PAH concentrations 340, 3960 or 8820 ng/g dw, respectively) to determine subsequent alterations in behavioral displays. Compared with control fish, the aggressive display of "opercular flaring" was significantly increased (P < 0.03, n = 14-16) in oil-exposed fish. Bubble nest building, as well as testis and brain mass, were significantly reduced in treated fish (P < 0.05). Hematocrit of treated groups was increased significantly (P < 0.02) from 21% in control fish to ∼27% in both oil exposure groups. Dietary exposure over a 4-week period to low, relevant levels of crude oil thus leads to an increase in aggressive behavioral displays, a decrease in reproductive activity and additional morphological changes.

Paternal exposure to environmental 17-alpha-ethinylestradiol concentrations modifies testicular transcription, affecting the sperm transcript content and the offspring performance in zebrafish

The synthetic estrogen 17-α-ethinylestradiol (EE2), a major constituent in contraceptive pills, is an endocrine disrupting chemical (EDC) present in the aquatic environment at concentrations of ng/L. Developmental exposure to these low concentrations in fish can induce several disorders. Zebrafish (Danio rerio) is a perfect organism for monitoring the effects of environmental contaminants. Our hypothesis is that changes promoted by EE2 in the germ line of male adults could be transmitted to the unexposed progeny. We exposed male zebrafish to 2.5, 5 and 10ng/L of EE2 during spermatogenesis and mated them with untreated females. Detailed progeny development was studied concentrating to survival, hatching and malformations. Due to the high incidence of lymphedemas within larvae, we performed qPCR analysis of genes involved in lymphatic development (vegfc and vegfr3) and endothelial cell migration guidance (cxcr4a and cxcl12b). Estrogen receptor (ER) transcript presence was also evaluated in sperm, testis and embryos. Progenies showed a range of disorders although at a low incidence: skeletal distortions, uninflated swimbladder, lymphedema formation, cartilage deformities and otolith tethering. Swimming evaluation revealed less active locomotion. All these processes are related to pathways involving ERs (esr1, esr2a and esr2b). mRNA analysis revealed that environmental EE2 causes the up-regulation of esr1 an esr2b in testis and the increase of esr2b transcripts in sperm pointing to a link between lymphedema in embryos and ER expression impairment. We demonstrate that the effects induced by environmental toxicants can be paternally inherited and point to the changes on the sperm transcriptome as the responsible mechanism.