Genetics and Cytology

Insect genetics has been a fruitful field and has contributed a great deal to this field of general science, particularly through studies on Drosophila. Much of this success is attributable to the ease with which many insects are maintained in the laboratory, their rapid turnover of generations, diversity of phenotypic expressions of gene effects, and in many cases, giant, well-marked chromosomes.

The genetic control of a large number of particular insect characteristics has been elucidated, such as the distribution of different types of hairs, color patterns, resis-

tance to insecticides, and wing venation. Gross changes in Lepidoptera wing color patterns are known to be determined by simple gene differences (Robinson 1971).

Sex in insects is basically determined by the production of different gametes, although epigenetic factors, such as hormones, are also important (Langé 1970). Sex chromosomes may be involved, a variety of combinations being found. Males heterozygous XY and XO and females homozygous XX is the usual situation. The reverse is true of Lepidoptera and Trichop-tera. In Hymenoptera, fertilized eggs develop into females, unfertilized eggs into males, the latter therefore being haploid individuals.

Genotype and gene frequencies are properties of populations rather than of individual insects. Their behavior is important to the understanding of evolutionary processes when it is realized that it is shifts in their frequency, either randomly (genetic drift), by mutation, selection, or external events, that lead to speciation and higher order phylogenetic changes. A classic case of the latter is the increase to normalcy of melanism in populations of European moths living in industrial environments where heavy soot pollution darkens their resting substrates (Kettlewell

1973). No melanics of this type are yet known in Latin America.

Mutations are easily induced in insects by means of radiation and chemicals. The former is even used routinely to create sterile individuals for mass release in genetic control schemes (Pal and Whitten

The mode of gene operation is also becoming known in insects. In the giant chromosomes of fly larvae, characteristic swellings, forming after natural hormones contact the cell, appear to indicate activity of specific genes.

Genetic work with other terrestrial arthropod groups aside from insects has lagged behind work with insects.

References

Kettlewell, H. B. D. 1973. The evolution of melanism. Clarendon, Oxford.

Langé, G. 1970. Relations entre le déterminisme génétique du sexe et la contrôle hormonal de sa différentiation chez les arthropodes: Comparaison avec les vertébrés. Ann. Biol. 9: 189-230.

Pal, R., and M. J. Whitten. 1974. The use of genetics in insect control. EJsevier/Norrh-Holland, Amsterdam.

Robinson, R. 1971. Lepidoptera genetics. Per-gamon, Oxford.

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