An Analysis of the Mode of Gene Action Affecting Pupa Weight in TRIBOLIUM CASTANEUM

Triple-testcross experiments (Kearsey and Jinks 1968) were employed to investigate the mode of gene action affecting pupa weight in Tribolium castaneum. Their experimental design involves two inbred lines, the F₁ progeny and a segregating population derived from the cross of the inbred lines. In the present experiments, four segregating populations were used. These populations included the F₂ generation, a select line (SEL) and two relaxed select lines (RSI and RSII). In addition, all possible reciprocal crosses were made among the RSI, RSII, and SEL populations. It was observed that: (1) additive, dominant and epistatic gene effects all made significant contributions to the pupa weight of the progeny from all four segregating populations; (2) there was no evidence of either accumulation of epistasis as a result of selection in the SEL population or decline in epistasis as a result of removing selection pressure from the RSI and RSII populations; and (3) significant negative heterosis and maternal effects contributed to the pupa weight of the crossbred progeny of the RSI, RSII and SEL populations.     

Growth Rate, Body Composition, Cellular Growth and Enzyme Activities in Lines of TRIBOLIUM CASTANEUM Selected for 21-Day Pupa Weight

Growth rate, body composition, cell number, cell size, and the activity of four dehydrogenase enzymes were studied from 10 to 25 days of age in one control (1C) and three lines (3, 9, 10) of Tribolium castaneum that had been subjected to long term selection for large 21-day pupae weight.-Selected lines were two- to three-fold larger in size than the control line throughout development. No major differences in percent of protein were detected among the lines but at any particular age, the selected lines were found to have a higher fat content than the control line. The differences in fat content were closely correlated with development such that all the lines reached very similar levels of percent of fat just prior to pupation. Water content showed an inverse relation to percent of fat.-Selection was observed to have caused major changes in the cellular response to growth. The selected lines had an average of from 17% to 48% larger cells (measured as protein/DNA) and were found to have from 37 to 62% more cells (measured as total DNA) than the control line at all ages from 10 to 19 days of age. In addition, the selected lines had a higher RNA content at all ages studied and higher RNA:DNA ratios at the young ages. In contrast the enzyme activities of ICDH and LDH were 60% lower. The results are interpreted as indicating that a more efficient metabolic machinery had evolved in the rapidly growing selected lines.     

Food Consumption, Feed Efficiency, Metabolic Rate and Utilization of Glucose in Lines of TRIBOLIUM CASTANEUM Selected for 21-Day Pupa Weight

Growth rate, body composition, cell number, cell size, and the activity of four dehydrogenase enzymes were studied from 10 to 25 days of age in one control (1C) and three lines (3, 9, 10) of Tribolium castaneum that had been subjected to long term selection for large 21-day pupae weight.—Selected lines were two- to three-fold larger in size than the control line throughout development. No major differences in percent of protein were detected among the lines but at any particular age, the selected lines were found to have a higher fat content than the control line. The differences in fat content were closely correlated with development such that all the lines reached very similar levels of percent of fat just prior to pupation. Water content showed an inverse relation to percent of fat.—Selection was observed to have caused major changes in the cellular response to growth. The selected lines had an average of from 17% to 48% larger cells (measured as protein/DNA) and were found to have from 37 to 62% more cells (measured as total DNA) than the control line at all ages from 10 to 19 days of age. In addition, the selected lines had a higher RNA content at all ages studied and higher RNA:DNA ratios at the young ages. In contrast the enzyme activities of ICDH and LDH were 60% lower. The results are interpreted as indicating that a more efficient metabolic machinery had evolved in the rapidly growing selected lines.     

Genetic Modification of Recombination Rate in TRIBOLIUM CASTANEUM

Asymmetrical responses were obtained in a replicated study of 15 generations of two-way selection for recombination rate between the ruby (rb) and jet (j) loci in Tribolium castaneum. Recombination rates in the two replicate high lines increased from an average of 0.22 in the base populations to an average of 0.42 at generation 15. Recombination rate pooled over the 15 generations of selection in each low line was significantly less than the control but there was no clear downward trend in response to selection for decreased recombination rate. The realized heritabilities were 0.16 +/- 0.03 and 0.17 +/- 0.02 in the two high lines, and were not significantly different from zero in the two low lines. Selection was based on crossing over in cis females only; however, rates measured in cis males after 12 generations showed the same response patterns as female rates. Similar response patterns were also determined for recombination measured in trans males and females at generation 18 following three generations of relaxed selection. The distribution of recombination rates measured in backcross beetles [(H X L) X H and (H X L) X L] at generation 12 indicated polygenic control with those genes decreasing recombination rate being dominant. Detailed analysis of recombination rates in F1's produced by interline crosses at generation 15 confirmed the directional dominance findings. Under a polygenic model of recombination modifiers in which low recombination is dominant to high, average recombination rates will increase as inbreeding progresses, thus providing a mechanism for the production of new gene combinations in small populations.     

THE EFFECT OF COEXISTENCE ON COMPETITIVE OUTCOME IN TRIBOLIUM CASTANEUM AND TRIBOLIUM CONFUSUM

We describe an experiment exploring the effects of coexistence and population differentiation on the competitive outcome of two species of Tribolium flour beetles, T. castaneum and T. confusum. The only manipulation was whether the populations used in the competitive phase of the study were raised initially in mixed-species communities, single-species populations, or in the standard culture conditions used to maintain stocks in the laboratory. Any treatment effects observed were due to natural selection acting within populations and genetic drift. In the competitive phase, we examined 10 mixed-species communities and 10 pairs of single-species populations. We replicated each community 15 times to provide an assessment of the distribution of competitive outcome. Statistical analysis demonstrates the lineages within the treatments became highly differentiated for all measures of competitive outcome: the outcome of competition (which species won), time to extinction of one of the competing species, and census history. The fraction of the variance that is among lineages has been referred to as the group or community heritability. All of these measures of competitive outcome had high community level heritabilities indicating that competitive ability would evolve rapidly as a result of group or community level selection. In contrast, competitive outcome was not affected by whether the two species had coexisted prior to the competitive phase. This indicates that the outcome of competition was not systematically changed by processes acting within the two species communities.     

Repair of X-Irradiation Damage in TRIBOLIUM CASTANEUM Strains Selected for Differences in Developmental Rates

Lines previously selected for different speed of development were found to differ in their spontaneous and induced wing abnormalities after single exposure to X-ray (10 KR) in Tribolium castaneum at 33 degrees and 70% relative humidity. At the time of irradiation the mean developmental time (from egg to first day pupa) was 16.9, 18.8, 22.8 and 18.4 days for fast, intermediate, slow and unselected lines, respectively. There was no difference between males and females in their responses. The spontaneous rate (angular values) of wing abnormalities was 5.3%, 11.2%, 3.5% and 7.2%, and the corresponding induced values were 16.7%, 20.5%, 11.7% and 25.2% for fast, intermediate, slow and unselected lines, respectively. The difference between the spontaneous (due to selection) and the induced (due to selection and irradiation) rates indicates that slowing down development will probably increase the efficiency of the repair of the irradiation damage. This difference (angular values) was 11.4%, 9.4% and 8.1% for fast, intermediate and slow lines, respectively. Increasing developmental time by one day resulted in repairing 0.56% of the radiation damage. These results indicate that both the genetic and the developmental state of the line influence both induction and repair of the X-irradiation damage. The relationship between development, cell cycle, protein synthesis and repair is discussed.