Qianqian Zhou

Background: The Polyplacophora class, which includes all chitons, is distinguished by its unique eight-piece interlocking armor, showcasing a vast diversity in marine environments. However, the detailed evolutionary relationships within the Chitonidae family remain largely unknown. The mitochondrial genome is essential for understanding these relationships, but there has been a significant lack of such genomic information, especially for the Liolophura genus. Methods: We generated the first mitogenome of Liolophura japonica by assembling Illumina reads with GetOrganelle, polishing with Pilon, annotating genes with MitoZ and MITOS2, and inferring phylogeny from 13 concatenated protein-coding genes (PCGs) using MAFFT and IQ-TREE. Results: The mitogenome is 15,209 base pairs long and includes 13 protein-coding genes, 22 transfer RNAs, and 2 ribosomal RNAs. The mitogenome exhibited a slight AT bias common in Chitonidae and showcased structural uniqueness with no control region found. Notably, all protein-coding genes demonstrated evidence of purifying selection, with Ka/Ks ratios below 1, highlighting evolutionary conservation. Phylogenetic analysis reveals a close relationship between L. japonica, Acanthopleura loochooana Broderip & Sowerby 1829, and Acanthopleura vaillantii Rochebrune, 1882, potentially warranting future taxonomic re-evaluation. This research emphasizes the crucial role of mitochondrial genomes in mollusk phylogeny and sets the stage for advanced genetic studies within this group. Conclusions: The significance of this study lies in its contribution to understanding the mitochondrial genome of L. japonica, a key species within the Polyplacophora class. By analyzing its mitogenome, we aim to enhance our understanding of evolutionary processes in chitons and other mollusks.

The extent and ecological significance of intraspecific diversity within marine microbial populations is still poorly understood, and it remains unclear if such strain-level microdiversity will affect fitness and persistence in a rapidly changing ocean environment. In this study, we cultured 11 sympatric strains of the ubiquitous marine picocyanobacterium Synechococcus isolated from a Narragansett Bay (Rhode Island, USA) phytoplankton community thermal selection experiment. Despite all 11 isolates being highly similar (with average nucleotide identities of >99.9%, with 98.6-100% of the genome aligning), thermal performance curves revealed selection at warm and cool temperatures had subdivided the initial population into thermotypes with pronounced differences in maximum growth temperatures. Within the fine-scale genetic diversity that did exist within this population, the two divergent thermal ecotypes differed at a locus containing genes for the phycobilisome antenna complex. Our study demonstrates that present-day marine microbial populations can contain microdiversity in the form of cryptic but environmentally-relevant thermotypes that may increase their resilience to future rising temperatures. Significance: Numerous studies exist comparing the responses of distinct taxonomic groups of marine microbes to a warming ocean (interspecific thermal diversity). For example, Synechococcus , a nearly globally distributed unicellular marine picocyanobacterium that makes significant contributions to oceanic primary productivity, contains numerous taxonomically distinct lineages with well documented temperature relationships. Little is known though about the diversity of functional responses to temperature within a given population where genetic similarity is high (intraspecific thermal diversity). This study suggests that understanding the extent of this functional intraspecific microdiversity is an essential prerequisite to predicting the resilience of biogeochemically essential microbial groups such as marine Synechococcus to a changing climate.