Meiotic self-pairing of the Psalidodon (Characiformes, Characidae) iso-B chromosome: A successful perpetuation mechanism

B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)15 microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.

School formation characteristics and stimuli based modeling of tetra fish

Self-organizing motion is an important yet inadequately understood phenomena in the field of collective behavior. For birds flocks, insect swarms, and fish schools, group behavior can provide a mechanism for defense against predators, better foraging and mating capabilities and increased hydro/aerodynamic efficiency in long-distance migration events. Although collective motion has received much scientific attention, more work is required to model and understand the mechanisms responsible for school initiation and formation, and information transfer within these groups. Here we investigate schooling of black tetra (Gymnocorymbus ternetzi) fish triggered by startle stimuli in the form of approaching objects. High-speed video and tagging techniques were used to track the school and individual members. We then measured several variables including reaction times, group formation shapes, fish velocity, group density, and leadership within the group. These data reveal three things: (1) information propagates through the group as a wave, indicating that each fish is not reacting individually to the stimulus, (2) the time taken for information to transfer across the group is independent of group density, and (3) information propagates across large groups faster than would be expected if the fish were simply responding to the motion of their nearest neighbor. A model was then built wherein simulated fish have a simple 'stimuli/escape' vector based on a hypothetical field of vision. The model was used to simulate a group of individual fish with initial conditions, size, and stimuli similar to the biological experiments. The model revealed similar behavior to the biological experiments and provide insights into the observed patterns, response times, and wave speeds.