# Genome-Wide Analysis Reveals Expansion and Positive Selection of Monocarboxylate Transporter Genes Linked to Enhanced Salinity and Ammonia Tolerance in Sinonovacula constricta

## TL;DR

This study identifies and analyzes monocarboxylate transporter genes in the clam Sinonovacula constricta, showing their expansion and role in adapting to environmental stress like salinity and ammonia.

## Contribution

The study reveals the evolutionary expansion and functional roles of MCT genes in mollusk environmental tolerance through genome-wide and transcriptome analyses.

## Key findings

- 16 ScSMCTs and 54 ScMCTs were identified in S. constricta, showing significant expansion compared to vertebrates.
- MCTs play a major role in responding to salinity and ammonia stress, with distinct expression patterns observed.
- Positive selection sites in MCTs suggest adaptive evolution linked to environmental stress tolerance.

## Abstract

Monocarboxylate transporter genes facilitate the transport of lactate, pyruvate, ketone bodies, etc., crucial for metabolic dynamic balance and intracellular pH regulation. To explore the roles of monocarboxylate transporter genes in the environmental tolerance of Sinonovacula constricta, a comprehensive genome-wide identification, phylogenetic evolution and expression analysis were conducted. Here, our genome-wide analysis identified 16 sodium-coupled monocarboxylate transporter genes (ScSMCTs) and 54 proton-coupled monocarboxylate transporter genes (ScMCTs) in S. constricta. Comparative genomic analysis revealed significant expansion of ScSMCTs and ScMCTs in mollusks compared to vertebrates, driven by tandem repeats and dispersed duplications. Transcriptome and temporal expression analyses demonstrated distinct roles of ScSMCTs and ScMCTs in response to salinity and ammonia stress, with MCTs playing a predominant role in abiotic stress responses. This study provides insights into the evolutionary expansion and functional divergence of monocarboxylate transporter genes in mollusks, offering a foundation for further research on their mechanisms in S. constricta.

Our previous genome analysis of Sinonovacula constricta revealed an expansion of the monocarboxylate transporter gene family, which is crucial for metabolic dynamic balance and intracellular pH regulation. To further elucidate the role of these expanded MCT genes in response to variable environmental conditions, we conducted a comprehensive genome-wide identification, phylogenetic evolution and expression analysis. In this study, 16 sodium-coupled monocarboxylate transporter genes (designated as ScSMCTs) and 54 proton-coupled monocarboxylate transporter genes (designated as ScMCTs) were identified from the S. constricta genome. The results of gene number comparison indicated significant expansion of ScSMCTs and ScMCTs in mollusks compared to vertebrates, likely due to tandem repeats and dispersed duplications in S. constricta. The syntenic analysis demonstrated that the razor-clam MCT genes had the highest number of homologous gene pairs with Meretrix meretrix. The phylogenetic tree showed that MCT and SMCT proteins were distinctly clustered in two large branches. Moreover, positive selection analysis revealed three positive selection sites in the MCT amino acid sequences sites. Multi-transcriptome analyses and the temporal expression patterns displayed that ScSMCTs and ScMCTs play distinct roles in response to salinity and ammonia stressors. It is worth noting that the majority of these genes involved in abiotic stresses belong to MCTs. Overall, our findings revealed the important roles of ScSMCTs and ScMCTs under abiotic stress, and provided valuable information for the evolution of this family in mollusks, as well as a theoretical basis for the further study of the mechanism and function of this gene family in S. constricta.