Deer mice Peromyscus maniculatus Normally they are dark in color, but members of this species that live in an area with very light soil (Sandy Hills in Nebraska) are colored lighter than their relatives. This helps them hide from birds of prey. Molecular genetic analysis showed that this adaptation arose less than 10,000 years ago as a result of a single mutation in the gene Agoutiaffecting hair pigmentation. This is one of the few cases when the evolution of an adaptive (useful) trait in natural conditions has been “deciphered” at all levels - from ecological to molecular.
The evolution of adaptations is one of the central themes of biological research. Despite the increased interest of scientists in this phenomenon, so far it has been possible in all details there are not many cases of evolution of adaptive traits in nature to trace and describe. There are a number of textbook examples, such as the change in beak length in Galapagos finches depending on the availability of certain seeds and interspecific competition (Grant & Grant, 2006), the emergence of antifreeze proteins in the blood of Antarctic fish (Chen et al., 1997), industrial melanism (see industrial melanism, Berry, 2008) and many others, however, in most cases, certain nuances of the complex evolutionary process of an adaptive trait remain undeciphered. Therefore, the emergence of a new example of a thoroughly studied and "decomposed" act of adaptive evolution is an important scientific event.
However, white-footed or deer mice from the Sand Hills in Nebraska (Sand Hills, Nebraska) with their light camouflage coloration became a textbook example of the evolution of adaptive traits long before scientists were able to decipher the molecular genetic basis of this adaptation. This was facilitated by two circumstances. Firstly, the adaptability of light color is beyond doubt: birds of prey see a dark mouse much better against a light background than a light one. Second, the Sand Hills are a relatively young geological formation: they formed after the retreat of the glacier about 10,000 years ago. This suggests that we are dealing with an adaptation that appeared - on an evolutionary timescale - quite recently. Therefore, references to Sand Hills mice can be found in textbooks and popular books on evolution alongside tales of industrial melanism, Darwinian finches, and antifreeze proteins. However, to complete the picture, it was very important to find out the genetic basis of this adaptation. This is precisely the goal set by the American evolutionary geneticists from Harvard and California Universities, who published their results in the latest issue of the journal Science.
The color of the mammalian coat depends on the distribution of two pigments: black-brown eumelanin and red phaeomelanin. The cells responsible for hair coloring (melanocytes) can alternately synthesize one or the other pigment as the hair grows. As a result, the hair is striped. Often the tip and base of the hair are dark (eumelanin), and in the middle there is a more or less wide light (pheomelanin) stripe. This is exactly the case with deer mice. As it turned out, the light coloration of mice from Sandy Hills is due to the fact that their pheomelanin strip is much wider than that of typical (dark-colored) representatives of this species living in areas with dark soil.
Having established this fact, scientists immediately suspected that the evolution of the protective color could be associated with the gene Agouti... A signaling protein encoded by this gene instructs melanocytes to synthesize pheomelanin instead of eumelanin (see Agouti signaling peptide). The effect of mutations in this gene on coat color has been studied in detail in house mice (Mus musculus). Known to be gene-disabled mutants Agouti are black in color, and increased gene activity leads to a very light color. It is also known that the "light" alleles (variants) of the gene Agouti usually dominant in relation to "dark" (this means that when crossing dark and light mice from pure lines, the first generation of offspring will be completely light, and in the second generation for every three light mice there will be one dark one).
To test if the light coloration of the Sandy Hills mice is indeed related to the gene Agouti, scientists crossed them with "normal" dark relatives (while all the offspring in the first generation turned out to be light), as well as with completely black mutants in which the gene Agouti disabled. The results of these crosses and a number of other tests allowed the authors to show that the light coloration of the Sand Hills mice is indeed due to a dominant mutation in the gene Agouti (and not in any other gene associated with coat color).
Further experiments showed that the direct result of the mutation is an increased gene activity Agouti in the first week of life, mice, that is, during the period when their wool grows. Maximum activity Agouti falls on the fourth day of life in both normal and light mice, but the absolute value of this activity (measured by the number of messenger RNAs read from the gene) in light specimens from Sandy Hills is several times higher than in normal representatives of this species.
The next task was to find out the nature of the mutation, that is, to determine exactly what changes in the nucleotide sequence of the gene Agouti responsible for light coloration. For this, the authors sequenced (determined the nucleotide sequence) of the gene Agouti in dark and light deer mice from their laboratory lines, as well as in 90 wild mice caught in the border zone at the edge of the Sand Hills, where both dark and light individuals are found. As a result, about 20 polymorphic sites were identified, that is, such gene regions that are not the same in all individuals. Currently, many very complex and effective methods of statistical analysis of nucleotide sequences have been developed, which allow, in particular, to detect traces of the action of positive (driving) selection on certain parts of genes and to distinguish useful (adaptive) DNA changes that were supported by selection from neutral ones. (not useful and not harmful) changes that spread in the population in a random way, without selection. By applying these techniques to gene sequences Agouti deer mice, the authors concluded that the key mutation that caused the mice to acquire a light color most likely consisted of the loss of three nucleotides encoding the amino acid serine. Changes in the remaining 19 polymorphic sites could also contribute, but less significant.
How is the loss of one amino acid from the signaling protein encoded by the gene Agouti, leads to the activation of the synthesis of this protein in the first week of life in mice - this is still unknown. But it was possible to show that this mutation, most likely, appeared and began to spread in the population of deer mice quite recently - much later than the glacier retreated and the Sand Hills with their light soil were formed. The results of statistical tests testify in favor of the youth of this mutation. In particular, it turned out that the "light" variants of the gene (in which the aforementioned three nucleotides are absent) vary much less at the rest of the polymorphic sites than the "dark" variants. This would not have been the case if this mutation (a drop of three nucleotides) existed in the population as neutral long before Sand Hills appeared and it became useful.
The study showed that the rapid formation of new adaptations can occur not only on the basis of preexisting genetic variability, but also due to new mutations that appear after the “need arose” for them. At the moment when the environmental conditions (and hence the direction of selection acting on the population) suddenly change, the population may not be "suitable" genetic variants that were neutral until now, but have now become useful. Most likely, the light color was not neutral, but definitely harmful to deer mice, while they lived only in areas with dark soil. But when the Sand Hills formed - a territory with light soil suitable for the life of deer mice - the situation immediately changed, and the light-colored mutants that occasionally appeared in the population, which selection had so far mercilessly weeded out, got their chance.
In general, as we can see, this study did not give anything fundamentally new for the evolutionary theory. The evolution of the protective coloration in deer mice occurred completely "according to the textbook", in strict accordance with generally accepted views. The main value of this work lies in the fact that it once again - and very clearly - confirmed the effectiveness of the classical "neo-Darwinian" model of the evolution of adaptations. After all, it is also useful to do this from time to time, and not only to refute dogmas.
A source: Catherine R. Linnen, Evan P. Kingsley, Jeffrey D. Jensen, Hopi E. Hoekstra. On the Origin and Spread of an Adaptive Allele in Deer Mice // Science... 2009. V. 325. P. 1095-1098.
Ⓘ Black-eared Hemispingus
Black-eared Hemispingus is a species of birds from the tanager family. Birds live in subtropical and tropical mountain humid forests, at an altitude of 1200 - 3050 meters above sea level. Body length 14 - 15 cm, weight about 14.5 grams.
There are three subspecies:
- Hemispingus melanotis berlepschi - on the eastern slopes of the Peruvian Andes from the Amazonas south to northern Cuzco,
- Hemispingus melanotis castaneicollis - Eastern slopes in southeastern Peru from Puno south to central Bolivia west of Santa Cruz.
- Hemispingus melanotis - Andes of western Venezuela in the south of Tachir south to the western slopes of the eastern Andes of Colombia from Santander and southeastern Boyaca south to the valley of the Magdalena River, not including Huila), as well as to the western eastern slopes near the city of Hardin isp. Jardin in Antioquia and the central Andes from Antioquia south to the eastern slopes at Nariño Colombia, although possibly found on the eastern slopes of the eastern Andes. In Ecuador, on the eastern slopes of the mountains south to Tungurahua and southeast to Podocarpus National Park,
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