Type II LDH/MDH Oxidoreductases in Pacific Oyster Magallana gigas (Thunberg, 1793): Gene Organization and Expression Patterns During Development and Across Tissues.

Type II LDH/MDH oxidoreductases represent a group of enzymes involved in diverse metabolic processes. In eukaryotes, AqE genes encoding these enzymes have been primarily documented in non-terrestrial plants and animals. Genomic analysis of the Pacific oyster Magallana gigas (Thunberg, 1793) revealed three AqE genes. The close genomic proximity of AqE2 and AqE3 suggests a relatively recent duplication event, supported by their conserved exon-intron organization, predicted protein properties, and phylogenetic clustering. Tree topology further indicates that AqE1 and AqE2/AqE3 homologs – residing on distinct chromosomes – diverged prior to the split of deuterostomes and protostomes. Despite their shared origin, M. gigas AqE homologs exhibit functional divergence post-duplication: AqE1 retains broad tissue/stage specificity (supposed housekeeping role), while AqE2 and AqE3 show enhanced developmental stage- and tissue-specific expression (supposed special functions). Given the absence of lactate dehydrogenase genes (LDH) in M. gigas and other bivalves, a hypothesis is proposed that AqE gene products may participate in lactate-pyruvate interconversion. These findings establish a foundation for future studies on type II LDH/MDH oxidoreductase evolution, including functional characterization of AqE protein products and analysis of their regulatory elements.