| Wing polymorphism has been found in many insect orders, and studies of wing polymorphism are very important in revealing the evolutionary mechanism of insect wings, improving our understanding of evolutionary strategies of life history traits and maintenance of polymorphism. In this study, geographical variation and genetic control of wing dimorphism have been studied in cricket species Velarifictorus aspersus Walker. Two genetic stocks of V. asperses were established after selection for 10 generations. One stock was nearly pure breeding for the long-winged (LW) phenotype (percentage of LW adults was about 95% ) while the other stock was pure-breeding for the short-winged (SW) phenotype (percentage of SW adults was about 90%) when raised under standard conditions. After then, effects of application of juvenile hormone Ⅲ (JH Ⅲ) and precocene on wing development were tested, and JH Ⅲ titer developmental profile was compared between nymphs from LW and SW stocks for analyzing the mechanism of JH control of wing dimorphism. To test the hypothesis of trade-off between flight capability and reproduction under circumstance of flying, effect of flight on development of flight muscles and reproductive organs was examined, and fighting ability between LW and SW males were compared. Finally, JH control mechanism of physical trade-off between flight capability and reproduction have been investigated. Major results are as follows:1. Individuals from three geographic populations (Haikou (20°02’N), Zhanjiang (21°11’N) and Changsha (28°12’N)) were reared under 30 ℃, LD 10:14 h, LD 12: 12 h, LD 14:10 h, and LD 16:8 h, respectively. Percentages of LW adults were all about 90% in populations from Haikou and Zhanjiang irrespective of photoperiodic conditions. For population from Changsha, percentages of LW adults were significantly higher when nymphs were reared under long photoperiodic conditions (LD 14:10 h and LD 16:8 h) than that of nymphs reared under short photoperiodic conditions (LD 10:14 h and LD 12:12 h). In addition, percentages of LW adults were significantly higher in populations from low latitudinal areas (Haikou and Zhanjiang) than population from relatively higher latitudinal area (Changsha). These results suggest that wing dimorphism of Ⅴ. aspersus is geographical variable, and may be correlated with latitudinal changes of climate. After genetic selection over 10 generations the frequency of SW adults in a SW selected line increased dramatically, whereas it remained at a very low level in a LW selected line. However, no 100% pure strains were obtained from both genetic selected lines. This result indicates that wing dimorphism of Ⅴ. aspersus is under polygenetic control.2. Analysis of heamolyph samples and JH Ⅲ standard by Liquid chromatograph-mass spectrometer demonstrated existence of JH Ⅲ in Ⅴ. aspersus. Quantification of JH Ⅲ in heamolyph samples from nymphs that were genetically-determined to develop into LW or SW adults showed that JH Ⅲ titer was high at penultimate instar, but low at last instar. JH Ⅲ titers in presumptively LW nymphs at 0,4,8 d of penultimate instar, and 0,4,8 d of last instar was 26.1 ± 6.5,21.1 ± 4.8,28.6 ± 3.4, 20.1 ± 4.2,12.0 ± 3.0, and 10.8 ± 1.9 pg/μl, respectively. JH Ⅲtiters in presumptively SW nymphs at 0,4,8 d of penultimate instar, and 0,4,8 d of last instar was 25.3 ±5.1, 22.9 ± 4.7,33.6 ± 10.4,20.1 ± 3.6,15.6 ± 2.8, and 12.8 ± 2.3 pg/μl, respectively. High titer of JH Ⅲ in heamolyph suggests that JH Ⅲ may be primitive type of JH in Ⅴ. aspersus. JH titer change profile between penultimate and last instars suggests a role of JH Ⅲ in determination of nymphal ecdysis and adult emergency.3. Application of high dose of JH Ⅲ had a strong brachypterizing effect during penultimate instar and early half period of last instar, and increasing function time of JH Ⅲ by two applications resulted in a stronger brachypterizing effect, suggesting that application of JH Ⅲ is able to suppress wing development of Ⅴ. aspersus. However, precocene could not induce macropters and JH Ⅲ titer did not changed significantly between presumptively LW and SW nymphs. These results did not support the hypothesis of JH control of wing polymorphism. Therefore, whether brachypterizing of JH Ⅲ application was caused by increasing the titer of JH Ⅲ in heamolyph that directly determinate wing morphs, or by influencing titers of ecdysone, neurohormones and other physical factors which control development of wings is uncertain, and need further researches to be confirmed.4. Examination of the effect of flight time on trade-off between flight capability and reproductive development in V. aspersus showed that flight of 5 min did not promote reproductive development of LW adults, but flight of 30,60, or 120 min could promote reproductive development both in female and male crickets. Because almost all of LW adults have partially histolyzed fight muscles at 48 h after a long time flight, the effect of a flight of 30 min on flight muscle and reproductive development of intact LW insects were further investigated to exclude influence of flight muscle degeneration. The results showed that flight influenced reproductive development directly. It was suggested that flight time may serve as a signal for LW V. aspersus to switch from migration to reproduction, and trade-off between flight ability and reproduction may be attenuated when flight time reaches a critical threshold.5. Food consumption of SW adults was significantly higher than that of unflown LW adults or LW adults with 5 min flight, but similar to that of LW adults with 30,60, or 120 min flight, suggesting that difference of reproductive development may be positively correlated with their food consumption. Flight capable insects are presumptive dispersers, and they may fly over a long distance to find a suitable new habitat. Due to spatial heterogeneity, there might be a large area where food may not be sufficient between one habitat and another. This physiological trait would allow them to solve the problem and reach a suitable new habitat successfully. After settling down, they will need to consume more food in order to provide enough nutrients for rapid reproduction.6. Male V. aspersus was equipped with extremely long mandibles and very aggressive toward conspecific males, suggesting that a strong fighting ability may be favored in evolution of this species. Fighting types varied in different social environments, and aggressive level of fighting for mate was significantly higher than that of fighting for a territory. This result indicated that male V. aspersus may be able to make assessment of the value of the contested resource, and adjust their fighting strategy accordingly. SW males increased more biomass than did the LW males after emergency, suggesting that SW males were stronger than LW males. However, LW males had shorter latency to leave from refuges than SW males, indicating that LW males were bolder than SW males. SW males won significantly more fights than LW males when fighting for mate, but LW males could win more fights when fighting for territory. Flight could make LW males become more aggressive, and help them to win more fights in competition of territory. Given that LW and SW males have different life history strategies, e.g. LW males are dispersers, and incline to explore new territory, while SW males invest more energy in reproduction. LW and SW males may choose to evolve different traits correlated with fighting success, such that both of them would fulfill their life history strategies.7. For investigation of flight ability of LW female Ⅴ. aspersus, virgin LW females aged 1,3,5,7, and 9 days after emergency were hanged to fly for 4 h, and total flight time and maximum duration of a single flight were recorded. The results showed that total flight time of LW females aged 1,3,5,7,9 d was 20.9 ± 1.8,96.8 ± 12.3,142.5 ± 12.8,130.1 ± 11.2, and 107.3 ± 10.1 min, respectively, and percentage of animals flied longer than 30 min in a single flight was 0,17.5%,40%,35%, and 20%, respectively. These results indicate that flight ability of LW female V. aspersus is affected by age, and period of 5-9 days after emergency may be migration phase of this species. It is also suggested that percentage of migrant is low in Ⅴ. aspersus, and most of LW females are local fliers.8. Comparison of JH Ⅲ developmental profiles between LW and SW females at different time on day 7 showed that JH Ⅲ titer of LW females exhibited a daily rhythm with a dramatically increase at 1 h before lights off, while JH Ⅲ titer of SW females stayed at relatively high level consistently. To test whether a temporal increase of JH Ⅲ was correlated with flight, effect of prconece Ⅱ (PⅡ) application on flight duration of LW females was investigated in next day, and reversing effect of JH was also conducted by application of 100 μg JH Ⅲ to LW females after treated with 100 μg P Ⅱ immediately. Results showed that high dose (100 and 200 μg) P Ⅱ significantly inhibited flight behavior, but JH Ⅲ could partly reverse the effect of PⅡ. It is suggested that the JH Ⅲ titer may be positively correlated with flight. Comparison of JH Ⅲ titer between LW females after a long time flight and LW females without flight showed that JH Ⅲ increased significantly at the time when flight was over. This result gives direct evidence for requirement of high titer of JH Ⅲ in long duration of flight. JH Ⅲ titer decreased quickly after flight, and returned to normal level at 10 h after flight. After then, it increased gradually and lost daily rhythm just like the pattern of SW females. During the same time, ovaries of flown LW females developed very quickly. Above all, high titer JH Ⅲ regulate both of the flight and reproduction, and a temporal increase of JH Ⅲ promotes migration, while a long time high titer of JH Ⅲ induce fast reproductive development. |
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