Kaufmann, P. | bioRxiv Science

bioRxiv2023

Trial-to-trial similarity and distinctness of muscle synergy activation coefficients increases during learning and with a higher level of movement proficiency

Muscle synergy analyses are used to increase our understanding of motor control. Spatially fixed synergy vectors coordinate multiple co-active muscles through activation commands, known as activation coefficients. To better understand motor learning, it is crucial to know how synergy recruitment varies during a learning task and different levels of movement proficiency. Within one session participants walked on a line, a beam, and learned to walk on a tightrope - tasks that represent different levels of proficiency. Muscle synergies were extracted over all conditions and the number of synergies was determined through the knee-point of the total variance accounted for (tVAF) curve. We found that the tVAF of one synergy decreased with task proficiency (line < beam < tightrope). Additionally, trial-to-trial similarity and distinctness of synergy activation coefficients increased with proficiency and after a learning process. We conclude that precise adjustment and refinement of synergy activation coefficients play a crucial role in motor learning.

neuroscience↗

bioRxiv2023

Y-linked copy number polymorphism of target of rapamycin(TOR) is associated with sexual size dimorphism in seed beetles

The Y chromosome is theorized to facilitate evolution of sexual dimorphism by accumulating sexually antagonistic loci, but empirical support is scarce. Due to the lack of recombination Y chromosomes are prone to degenerative processes, which poses a constraint on their adaptive potential. Yet, in the seed beetle Callosobruchus maculatus segregating Y linked variation affects male body size and thereby sexual size dimorphism (SSD). Here we assemble C. maculatus sex chromosome sequences and identify molecular differences associated with Y-linked SSD variation. The assembled Y chromosome is largely euchromatic and contains over 400 genes, many of which are ampliconic with a mixed autosomal and X chromosome ancestry. Functional annotation suggests that the Y chromosome plays important roles in males beyond primary reproductive functions. Crucially, we find that, besides an autosomal copy of the gene target of rapamycin (TOR), males carry an additional TOR copy on the Y chromosome. TOR is a conserved regulator of growth across taxa, and our results suggest that a Y-linked TOR provides a male specific opportunity to alter body size. A comparison of Y haplotypes associated with male size difference uncovers a copy number variation for TOR, where the haplotype associated with decreased male size, and thereby increased sexual dimorphism, has two additional TOR copies. This suggests that sexual conflict over growth has been mitigated by autosome to Y translocation of TOR followed by gene duplications. Our results reveal that despite of suppressed recombination, the Y chromosome can harbour adaptive potential as a male-limited supergene.

evolutionary biology↗