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Due to its proposed basal position in the bilaterian Tree of Life, Acoela may hold the key to our understanding of the evolution of a number of bodyplan features including the central nervous system. In order to contribute novel data to this discussion we investigated the distribution of a-tubulin and the neurotransmitters serotonin and RFamide in juveniles and adults of the sagittiferid Symsagittifera roscoffensis. In addition, we present the expression pattern of the neuropatterning gene SoxB1. Adults and juveniles exhibit six serotonergic longitudinal neurite bundles and an anterior concentration of serotonergic sensory cells. While juveniles show an ''orthogonlike'' arrangement of longitudinal neurite bundles along the anterior-posterior axis, it appears more diffuse in the posterior region of adults. Commissures between the six neurite bundles are present only in the anterior body region of adults, while irregularly distributed individual neurites, often interconnected by serotonergic nerve cells, are found in the posterior region. Anti-RFamide staining shows numerous individual neurites around the statocyst. The orthogon-like nervous system of S. roscoffensis is confirmed by a-tubulin immunoreactivity. In the region of highest neurotransmitter density (i.e., anterior), the HMG-box gene SrSoxB1, a transcription factor known to be involved in neurogenesis in other bilaterians, is expressed in juvenile specimens. Accordingly, SoxB1 expression in S. roscoffensis follows the typical pattern of higher bilaterians that have a brain. Thus, our data support the notion that Urbilateria already had the genetic toolkit required to form brain-like neural structures, but that its morphological degree of neural concentration was still low.

Abstract:A long-standing question in molecular biology relates to why the testes express the largest number of genes relative to all other organs. Here, we report a detailed gene expression map of human spermatogenesis using single-cell RNA-Seq. Surprisingly, we found that 20 spermatogenesis-expressed genes contain significantly fewer germline mutations than unexpressed genes, with the lowest mutation rates on the transcribed DNA strands. These results suggest a model of 'transcriptional scanning' to reduce germline mutations by correcting DNA damage. This model also explains the rapid evolution in sensory-and immune-defense related genes, as well as in male reproduction genes. Collectively, our results indicate that widespread 25 expression in the testes achieves a dual mechanism for maintaining the DNA integrity of most genes, while selectively promoting variation of other genes.. CC-BY-NC-ND 4.0 International license It is made available under a (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/282129 doi: bioRxiv preprint first posted online Mar. 14, 2018; 2 Main Text:Human tissues and organs are distinguished by the genes that they express and those that they do not 1,2 . Tissues have transcriptomes of different complexities in terms of uniquely-expressed genes, as well as those genes expressed at differential levels [3][4][5][6] . One overarching goal in the life sciences is to characterize the specific transcriptomic signatures of all human tissues, and 5 ultimately each different cell type at the single-cell level 7 .In males, the testis is unique in comparison with somatic tissues in that it contains germ cells which pass the genetic information on to the next generation 8 . Interestingly, it has been known for many years that the testis stands out as having the most complex transcriptome with the highest number of expressed genes [9][10][11][12] . Widespread transcription in the testes has been 10 reported to account for an amazing expression of over 80% of all our protein-coding genes 10,11,13 , as well as across many other mammals 3,10 .Several hypotheses have been proposed to explain this observation. Widespread expression may represent a functional requirement for the gene-products in question 12 .However, other more complex organs such as the brain do not exhibit a corresponding number of 15 expressed genes despite the fact that they consist of a substantially greater number of distinct cell types 3,10,14-16 . Moreover, recent animal studies have shown that many testis-enriched and evolutionarily-conserved genes are not required for male fertility in mice 17 . A second hypothesis implicates leaky transcription during the massive chromatin remodeling that occurs throughout spermatogenesis 12,18,19 . However, this model predicts more expression during later stages of 20 spermatogenesis -when the genome is undergoing the most chromatin changes -contradicting ...