The world's fastest muscles reveal a surprising evolutionary history, challenging long-held assumptions about muscle function across vertebrates. A recent study, led by James Pease at The Ohio State University, has uncovered a complex web of molecular diversity that defies the conventional understanding of muscle mechanics.
The research began with an investigation into a unique muscle protein in birds known for rapid wing snaps. By comparing the mammal myosin gene layout with the chicken genome, the team discovered a significant discrepancy. Chickens possess a distinct set of genes associated with muscle movement, diverging from the expected mammalian blueprint.
This finding prompted a broader analysis, utilizing public genomic databases to trace the evolutionary history of myosin genes across 1,201 myosins from 119 species. The results were eye-opening, revealing that each major group of vertebrates has developed its own unique set of core skeletal muscle myosins, evolving independently over millions of years.
The study identified at least 50 new subfamilies of myosin genes, in addition to the 15 previously recognized ones. This diversity is attributed to a slow shuffling of genes, where copies are duplicated and lost, leading to each lineage holding a unique set of genetic cards from a shared deck.
One of the most intriguing findings is the variation in the molecular machinery used to build fast and slow muscles. Mammals, for instance, rely on two distinct types of muscle fibers: slow-twitch fibers for sustained posture and fast-twitch fibers for rapid bursts. However, this binary classification does not apply to other vertebrates.
Birds, lizards, fish, and mammals employ different molecular mechanisms to create fast and slow muscles. This indicates that the molecular basis for fast-twitch and slow-twitch muscles in mammals is not universally applicable across the animal kingdom.
The study also highlights the specialization of certain myosins in specific animal groups. For example, the western diamondback rattlesnake uses distinct myosins for its head, midsection, and tail muscles, with one particular myosin dominating the gene expression in the tail segment responsible for the rattle.
Similarly, superfast muscles in various animals, such as hummingbirds and bats, rely on different myosins, suggesting that speed has been reinvented multiple times through diverse molecular pathways.
A key factor in this diversity is the variation in two small surface loops within the myosin protein. These loops play crucial roles in determining the speed of chemical fuel processing and the tightness of the grip between myosin and actin. Different animals have evolved unique loop designs to build fast muscles, indicating that there is no single 'winning' shape.
The study's authors are cautious about attributing specific reasons for this diversity, as adaptation is challenging to prove. However, they suggest that the presence of multiple molecular subtypes is unlikely without some form of selective or adaptive process driving their evolution.
In essence, this research demonstrates that evolution can preserve essential functions while simultaneously transforming the molecular components that enable them. It opens up new avenues for understanding muscle biology and the intricate relationship between genetic diversity and functional specialization in the animal kingdom.
