Major Histocompatibility Complex Immunogenetic Diversity Differs Substantially Across Sea Turtle Species and Genomic...

Area of Science:

• * Evolutionary genetics
• * Immunology
• * Marine biology

Background:

• * Major histocompatibility complex (MHC) immune loci are crucial for understanding functional genetic diversity shaped by natural selection in wild populations.
• * High MHC diversity within individuals and populations is typically favored by balancing selection, persisting across speciation events.
• * Some vertebrate groups exhibit reduced MHC diversity or loss of MHC gene classes, necessitating further investigation in diverse taxa.

Purpose of the Study:

• * To investigate interspecific MHC (class I and class II) genetic diversity in four sea turtle species.
• * To explore the role of balancing selection in maintaining MHC diversity in sea turtles.
• * To identify variations in MHC diversity and evolutionary pressures among sea turtle species and gene copies.

Main Methods:

• * Sequencing of class I and class II MHC genes from over 300 individuals across four sea turtle species: loggerhead, green, leatherback, and Kemp's ridley.
• * Analysis of allele counts, nucleotide diversity, and evidence of selection (diversifying and positive selection).
• * Codon usage analyses to infer the maintenance of shared alleles through speciation.

Main Results:

• * Recovery of 141 class I and 308 class II functionally distinct MHC alleles, with many shared among species, suggesting maintenance through speciation.
• * Evidence of balancing selection, including high allele counts and signatures of diversifying and positive selection, indicating considerable MHC diversity in most sea turtles.
• * Notable exceptions include leatherback sea turtles (Dermochelys coriacea) with extremely low nucleotide and allelic diversity, and an atypical MHC class II gene copy with minimal variability across all species.

Conclusions:

• * Sea turtle MHC exhibits high allelic and nucleotide diversity, primarily driven by balancing selection.
• * Evolutionary pressures on MHC genes vary significantly between sea turtle species and even among different gene copies.
• * The low diversity in leatherback sea turtles and the atypical MHC class II gene copy warrant further investigation into their functional implications and evolutionary trajectories.