Chromosomal polymorphism in irradiated natural populations of Chironomus.

CHROMOSOMAL polymorphism occurs in many species of Drosophila and Chironomus. Evidence has been accumulated by BEARDMORE, DOBZHANSKY and PAVLOVSKY ( 1960), BATTAGLIA and SMITH (1962), and others that chromosomal polymorphic populations are superior in fitness to monomorphic populations. In most cases it is accepted that populations remain chromosomally polymorphic because the heterokaryotypes are superior in fitness to the homokaryotypes. The viabilities of the different chromosomal forms may be so delicately adjusted to the environment that subtle changes can have a marked effect on their frequencies. It is well known that the exposure of living organisms to ionizing radiation may result in genetic damage in the form of gene mutation and chromosome aberrations. However, the effect that ionizing radiation would have on balanced chromosomal polymorphism in natural populations is not clearly understood. SEECOF (in STONE et al. 1957) detected no increase of new chromosome rearrangements in heavily irradiated natural populations of Drosophila ananussag. CORDEIRO (1961) found two new inversions and a marked decrease in chromosomal polymorphism in isolated natural populations of D. willistoni that had received large releases of progeny from heavily irradiated parents. PAGET (1954) observed some new inversions at high frequencies in WALLACE and KING’S (1951 ) chronically irradiated laboratory populations of D. melunogaster. MOURAD ( 1962) utilized the phenomenon of balanced chromosomal polymorphism in caged populations of D. pseudoobscuru to show that irradiated chromosomes decline in frequency for a generation or two, but recover and establish a balanced equilibrium at a frequency below the controls. In MOURAD’S populations, when irradiated chromosomes were added to each generation, an equilibrium strikingly below the controls was reached. BLAYLOCK (1965) found several chromosomal aberrations unique to a population of Chironomus living in a stream contaminated by radioactive waste. Three paracentric inversions occurred at a relatively high frequency in this irradiated natural population. This report concerns a comparison of the chromosomal polymorphism produced by these three inversions in the irradiated population with a nonirradiated population. In addition, the finding of more unique aberrations in