Molecular and ecological signs of mitochondrial adaptation: consequences for introgression?

The evolution of the mitochondrial genome and its potential adaptive impact still generates vital debates. Even if mitochondria have a crucial functional role, as they are the main cellular energy suppliers, mitochondrial DNA (mtDNA) introgression is common in nature, introducing variation in populations upon which selection may act. Here we evaluated whether the evolution of mtDNA in a rodent species affected by mtDNA introgression is explained by neutral expectations alone. Variation in one mitochondrial and six nuclear markers in Myodes glareolus voles was examined, including populations that show mtDNA introgression from its close relative, Myodes rutilus. In addition, we modelled protein structures of the mtDNA marker (cytochrome b) and estimated the environmental envelopes of mitotypes. We found that massive mtDNA introgression occurred without any trace of introgression in the analysed nuclear genes. The results show that the native glareolus mtDNA evolved under past positive selection, suggesting that mtDNA in this system has selective relevance. The environmental models indicate that the rutilus mitotype inhabits colder and drier habitats than the glareolus one that can result from local adaptation or from the geographic context of introgression. Finally, homology models of the cytochrome b protein revealed a substitution in rutilus mtDNA in the vicinity of the catalytic fraction, suggesting that differences between mitotypes may result in functional changes. These results suggest that the evolution of mtDNA in Myodes may have functional, ecological and adaptive significance. This work opens perspective onto future experimental tests of the role of natural selection in mtDNA introgression in this system.

[Nucleus motorius medialis in the lumbosacral segment of the spinal cord in horses]

The studies carried out on 2 spinal cords of horses showed that cells of the medial motor nucleus (nucleus motorius medialis) are present in all neuromers of the lumbar and sacral segment of the spinal cord. It lies in the medial part of grey matter of the ventral column, neighbouring laterally and ventrally with cells of the lateral motor nucleus, whereas dorsally with cells of the nucleus of the ventral commissural horn. Along the nucleus numerous constrictions and intervals are found, which are connected with various numbers of nerve cells in particular cross-sections.

[Nucleus motorius lateralis in the lumbosacral segment of the spinal cord in horses]

Two medullae oblongatae of horses were cut into 15 microns cross-sections and stained according to the modified method of Nissel. The lateral motor nucleus lies in the lateral and median part of the ventral column of spinal cord grey matter. It adjoins medially nucleus motorius medialis of the spinal ventral column. Cells of this nucleus occur both along the whole lumbar and sacral segment of the spinal cord. In the lateral motor nucleus three cell groups are distinguished-median, basal and lateral. The latter is divided in some segments into subgroups-dorsal and ventral. Along the nucleus quite numerous constrictions and intervals are found, which are caused by various numbers of cells in particular cross-sections. Nucleus motorius lateralis is formed mainly of large and medium multipolar or single spindle cells.