Natural Heteroplasmy and Mitochondrial Inheritance in Bivalve Molluscs

Area of Science:

• Mitochondrial biology
• Genetics
• Evolutionary biology

Background:

• Heteroplasmy, the coexistence of multiple mitochondrial genome types, is typically unfavorable and impacts mitonuclear interactions.
• Doubly Uniparental Inheritance (DUI) is a unique, evolutionarily stable heteroplasmic system in Metazoa, found in bivalves, with distinct egg (F-type) and sperm (M-type) mitochondrial lineages.
• High amino acid sequence divergence (up to 52%) exists between F- and M-type mitochondrial proteins within the same bivalve species.

Purpose of the Study:

• To investigate mitochondrial heteroplasmy at the protein level within tissues, cells, and organelles for the first time.
• To assess the extent and expression of heteroplasmy in the DUI system of Ruditapes philippinarum.
• To examine the implications of findings for DUI models and germline development.

Main Methods:

• Immunolocalization of F- and M-type variants of three mitochondrially-encoded proteins.
• Analysis in germline and somatic tissues across different developmental stages of Ruditapes philippinarum.
• Investigation of heteroplasmy at the organelle level.

Main Results:

• Heteroplasmy was detected at the organelle level in undifferentiated germ cells (both sexes) and male somatic tissues.
• Gametes were found to be homoplasmic: eggs carried the F-type and sperm carried the M-type mitochondrial variant.
• Evidence suggests a meiotic drive mechanism maintains sex-specific mitochondrial variants during gametogenesis.

Conclusions:

• The study provides the first evidence of mitochondrial heteroplasmy at the protein level in a DUI system.
• A revised model for DUI is proposed, highlighting the role of meiotic drive in maintaining sex-specific mitochondrial inheritance.
• Findings contribute to understanding mitochondrial-nuclear interactions and germline development in bivalves.