Variazioni intrastagionali dello spotting e selezione in Maniola Jurtina L. (Lepidoptera, Satyridae)

Scali, Valerio ; Masetti, Massimo

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 58 (1975) no. 2, pp. 244-257

Résumé

Spot-distributions observed in Mamola jurtina during the emergence spell of both sexes do not show significant intraseasonal shifts in continental low colonies of central Italy, where an aestivation of the females occurs. Similarly, after the diapause, egg-laying females do not undergo any real spotting shift. Comparisons between the spot-distribution of female samples taken before and after aestivation show however, in certain years, significant spotting differences, the shifts always tending towards lower values. The calculated selection against highly spotted phenotypes during the imaginal diapause amounts in many instances to 65—70%. In mountain colonies, where no diapause occurs, male spotting does not usually show any significant intraseasonal change; female spotting, on the contrary, often shows significant shifts towards lower values as the flying activity procedes. Gonadal ripening analysis shows that egg-laying only starts when the lowering of the spot-distribution has already occurred. At Poggio, a colony of intermediate altitude, the spotting of the females, flying at different dates, has been correlated with their emergence time through the ascertainment of the gonadal condition of each specimen. It has been possible to detect that the selection against highly spotted adults is responsible for the lowering of the spotting among early emergers, but later on the maintaining of low spot-values or the further lowering is due to the relative increase in frequency of low spotted hatching females. The results here obtained suggest that this second mechanism is at work when intraseasonal shifts are produced in mountain colonies. The increase of low spotted specimens among late emergers seems rather due to selection activity on pre-imaginal instars than to differential growth-speed of the various presumptive spot-phenotypes. Intraseasonal shifts, however brought about, indicate the physiological role of the spot-genes and suggest the existence of balancing advantages and disadvantages of the different spot-genotypes during the insect life-cycle. From the data on the intraseasonal shifts it appears that the female sex is much more selected !than the male one; consistently with the sex-ratio changes these findings point out a more direct role of the females in the evolutionary adjustments of this butterfly.