Genetic control
Coloration is in large measure determined genetically. As mentioned earlier, the inheritance of colour in garden peas provided part of the basis for the pioneering studies of heredity by Mendel. These studies led Mendel to postulate the existence of discrete units of heredity that segregate independently of one another during the formation of reproductive cells. The studies also led to his discovery of the phenomenon of dominance. The basic units of heredity are now known as genes, and the variant forms of a given gene are termed alleles. Among species that reproduce sexually, an individual normally possesses a pair of alleles for any gene—one inherited from the female parent and one from the male parent. These two alleles are situated at corresponding loci on the paired chromosomes found in diploid cells—i.e., cells containing two similar sets of complementary chromosomes. Segregation of the alleles occurs during formation of reproductive cells, with the result that only one of the pairs enters each cell, which is called a haploid cell.
In his experiments Mendel crossed purple-flowered peas with white-flowered ones. The plants he used in these crosses were true-breeding for flower colour, meaning that the purple-flowered plants were descended for generations from only other purple-flowered plants, and that the white-flowered plants were likewise descended for generations from only other white-flowered plants. Because of these true-breeding characteristics, Mendel postulated that the original plants were homozygous for the trait of flower colour—in other words, that each plant carried a pair of identical heredity units (i.e., alleles) for this trait. When he crossed purple-flowered peas with white-flowered ones, he obtained a first filial (F1) generation in which all the offspring had purple flowers. He therefore deduced that the unit for purple (usually designated R) was dominant over the unit for white (r). Thus in the parental generation the purple-flowered plants can be designated RR (indicating that they are homozygous for the dominant allele), and the white-flowered plants can be symbolized as rr. The F1 plants were heterozygous for flower colour (Rr), but they expressed purple colour because of the complete dominance of the allele R over r.
Dominance may be incomplete, however; a crossing between homozygous red Japanese four-o’clocks (Mirabilis) and homozygous white ones yields heterozygous Rr offspring, which are all pink. A cross of the heterozygous pink generation of four-o’clocks with each other yields a second generation with the colour ratio of 25 percent red (RR), 50 percent pink (Rr), and 25 percent white (rr). This is because each of the parent (F1) plants produces equal numbers of R- and r-containing reproductive cells through segregation, and there is a random chance of either type of male haploid cell (gamete) fertilizing either of the two female types. For peas, on the other hand, the ratio resulting from a cross of parent (F1) plants is three purple (one RR and two Rr) to one white (rr) because of the dominance of R.
Although the principle of inheritance of colour and coloration patterns in all organisms is like that for the two plants described above, it is usually far more complex. Within the species population, a particular gene may have multiple alleles instead of two; thus numerous combinations within any individual may be possible; in addition, the coloration may depend upon genes at several sites. In this case either all pairs may segregate simultaneously and more or less independently into the gametes, or the genes may be linked in their inheritance by location on the same chromosome. Such possibilities, together with different degrees of dominance, result in tremendously complex hereditary bases for the genetic control of colour and colour patterns within many species. For a fuller treatment of these principles, see Genetics And Heredity, The Principles Of: Mendelian genetics.
Physiological control
The development of coloration often depends upon regulatory substances (hormones) secreted by endocrine glands. In birds the level of the hormone thyroxine determines the coloration of feathers and bill, although specific seasonal biochromes are often laid down under the influence of sex hormones, as in the beak of the starling, which turns from black to yellow in early spring. The variability in control among bird species is so great, however, that generalizations are impossible. Hormonally controlled colour changes also occur in mammals; for example, swellings in the genital areas that become pink due to vascularization during the reproductive season. The species specificity of coloration patterns, however, always depends on a genetically determined responsiveness of various target tissues to certain hormones.
Chromatophores occur in cephalopods, crustaceans, insects, fishes, amphibians, and lizards and are responsible for the most rapid colour changes. They allow conspicuous display of a biochrome by dispersing it in the chromatophore-bearing surface, or they conceal the biochrome by concentrating it into small areas. Chromatophores are of three kinds. The chromatophoric organs of cephalopods consist of an elastic sac filled with biochrome and controlled by a ring of radiating muscle fibres. These fibres contract in response to neural stimulation, thereby stretching the sac into a broad, thin disk. Chromatophoric syncytia occur in crustaceans, the movement of biochrome being due to the ebb and flow of cytoplasm through fixed tubular spaces that collapse when the cell is contracted and fill when the cell expands. Chromatophoric syncytia are hormonally controlled. Cellular chromatophores, the third kind, are found in vertebrates. In these cells melanic granules flow in stable cellular processes that maintain a fixed position, unlike the contracting and expanding processes of the syncytia. Control among vertebrates is varied: chromatophores of bony fishes are controlled by the autonomic nervous system; those of elasmobranch fishes (sharks and rays) and lizards are controlled by hormones and nerves; those of amphibians are regulated by hormones alone.
One animal may contain biochromes of several colours, commonly red, yellow, black, and reflecting white; prawns also have a blue biochrome. By appropriate migrations of biochromes, an animal can achieve substantial alterations in colour or shade for varying periods of time. In prawns, dispersion of blue and yellow yields green; unequal dispersion of biochromes over parts of the body produces patterns of coloration.
Rapid physiological colour changes are supplemented by morphological ones, the animal either gradually synthesizing or destroying biochromes, usually in an adaptive manner (see the section Coloration changes).
Frank A. Brown Edward Howland Burtt