Sulfur and Adaptive Phylogeography.

Phylogeography is an important multidisciplinary field of biological research that seeks to determine the geographic distribution of genealogical lineages (Avise et al., 1987). An essay by Martins and Domingues (2011) provides insight into fundamental concepts and the status of this discipline in Brazil. The article “Adaptive phylogeography: functional divergence between haemoglobins derived from different glacial refugia in the bank vole” by Petr Kotlík et al. (2014) is a pioneering study suggesting that phylogeographic patterns may result from adaptive processes involving competition and selection.

In this study, conducted with a small rodent approximately 10 cm in length — the bank vole (Myodes glareolus, formerly Clethrionomys glareolus) — which colonized Great Britain at the end of the last glacial period, the authors demonstrated that an initial population of the species, which had already spread throughout the region, was replaced in southern Great Britain (covering nearly all of England) by another population of the same species, originating from a different glacial refuge and already carrying an adaptive advantage.

A significant physiological difference between these two populations was the presence, in the second, of erythrocytes with three times greater antioxidant capacity. Experimental results indicated that this was due to the presence, in the erythrocytes of the second wave of colonizers, of a major hemoglobin component of the β52Cys type, instead of β52Ser, which was present in the partially replaced population. Structural and physicochemical studies showed that the thiol group of β52Cys is widely exposed on the molecular surface, has a low pKa, and reacts with diamide to produce extensive polymerization via disulfide bridges (-S-S-), as demonstrated by its complete reversal upon reaction with 2-mercaptoethanol, a reagent that reduces disulfide bonds.

The evolutionary significance of this thiol-rich hemoglobin in one of the bank vole populations is attributed to superior protection in situations involving reproductive cost, climatic stress, intraerythrocytic parasitoses, and diabetes — all conditions faced by populations of these rodents, as discussed by the authors. Thiol-rich hemoglobins are defined as those with eight or more cysteine residues per hemoglobin molecule. In vertebrates, the number of cysteines per hemoglobin ranges from zero to around eighteen. Crocodiles, sharks, and freshwater turtles are among the taxa with the highest content of cysteine residues (–SH group: thiol, sulfhydryl, mercaptan) in some of their hemoglobins, with some of these thiols fully exposed on the surface, making them highly reactive.

It is worth noting that thiol-rich hemoglobins occur in other taxa and vertebrate species. The reason for their evolutionary fixation is generally not clearly determined. However, the statement that “sulfur makes champions” seems largely justified, as resistance to extreme environmental conditions and exceptional physical performance are frequently observed in species bearing thiol-rich hemoglobins.