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Giant chromosomes, ecology, and adaptation in Chironomus Tentans Topping , Milton Stanlee


Ecological adaptation has occupied a central position in evolutionary theory for over 100 years; however, surprisingly little is known in detail of its significance. This study was undertaken to determine if chromosomal inversions of larvae of Chironomus tentans have any selective significance in nature and if they do, to define the relationship between genetic adaptation, as reflected by the frequencies of the inversions, and the ecological conditions to which the species mast be adapted. The procedure used was (1) to define the habitats potentially available to C. tentans, (2) to define the ecological conditions to which C. tentans is adapted, and (3) to define the relationship between inversion frequency and those ecological conditions to which C. tentans has been found to be adapted. The environmental properties of 32 saline lakes located in south-central British Columbia were studied. The lakes differed in total chemical concentration, ranging from very fresh to four or five times the concentration of sea water. Measurements of temperature, conductivity of the water, total dissolved solids, and pH, and of concentrations of dissolved oxygen, sodium, potassium, calcium, magnesium, carbonate, bicarbonate, chloride, and sulfate were made during different seasons at different depths in most of the lakes. Surface water samples were analyzed by a semi-quantitative technique for 28 trace elements. Selected samples of bottom muds were analyzed for substrate composition and percentage organic carbon content. Analysis of the environmental properties indicated that the chemical concentrations of the lakes varied both seasonally and with depth. Sodium, magnesium, carbonate-bicarbonate, and sulfate were the ions present at the greatest concentrations. The occurrence and abundance of larvae of C. tentans were determined with respect to the environmental properties of the lakes. C. tentans was found to occur in only 17 of the 32 lakes studied and each of the 17 lakes had conductivities within the range of from 500 to 4,500 micromhos. Further, C. tentans occurred only in the upper two meters of these lakes and was most abundant at the shallower depths of its occurrence. Both occurrence and abundance of C. tentans were correlated primarily with chemical concentration, although other environmental factors also were found to be important. Analysis of the frequencies of certain inversions present in chromosome 1 indicated that, populations present in different environments may have significantly different inversion frequencies, that the frequencies did not vary seasonally and were extremely stable over time periods as long as eight years, and that the inversion frequencies of sub-divisions of populations living within lakes do not vary significantly, Although C. tentans was found to show considerable ecological adaptation to lake water chemistry, no obvious relationship between the frequencies of the inversions of chromosome 1 and lake water chemistry could be detected. The studies of correlation of inversion frequencies with environmental properties and the results of a field experiment indicate that C. tentans is genetically adapted primarily to its co-occurrence with other chironomids. A lesser correlation with pH was also detected. Consequently, although genetic adaptation to environmental factors was found, that genetic adaptation was not to the major differences which occurred between the lakes. The unique and major contributions of this study are the demonstration that the inversions of chromosome 1 have selective significance in nature and that not all ecological adaptation is accompanied by parallel genetic adaptation (as reflected by inversion frequency). The clear implication is that although ecological adaptations may have evolutionary significance, not all ecological adaptations do.

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