THE ENVIRONMENTAL CONTROL OF POLYMORPHISM IN AN APHID 



Зависимость по.1иморфи;{ма у тлей от внешних факторов 



А. D. LEES 



(Agricultural Research Council Unit of Insect Physiology, Cambridge, England) 



The mechanisms controHing form determination in aphids are кполуп to 

 involve an important environmental component. Although, for example, sex 

 determination in these insects is considered to have a genetic basis, the environ- 

 ment usually exerts an important modifying influence. The fundatrices, Avhich 

 arise as the inevitable product of the fertilized egg, do not have this component. 

 But the production of the other female morphs is often closely regulated by 

 external physical or biotic factors. Recent work on the vetch aphid Megoura 

 viciae Buckton has shown that this statement applies to the apterous and alate 

 virginoparae and to the oviparae which together comprise the remaining 

 female morphs in this species. Nevertheless, the precise action of the environ- 

 ment cannot be defined лvithout reference to the reacting systems of the parents, 

 since the latter have differing potentialities as form-producers. These differing 

 capacities may go hand in hand with morphological differences or, as in the 

 case of virginoparae from young and old clones, they may not be associated 

 with any easily recognisable anatomical peculiarities. Some of these relation- 

 ships are considered in this paper. 



I have recently shown that the formation of virginoparae and oviparae 

 in long-established clones of Megoura is governed by photoperiod and temper- 

 ature (Lees, 1959). Apterous parents, which have been exposed since their 

 birth to short daily photoperiods at medium temperatures, produce only 

 oviparous daughters, whereas those exposed to long photoperiods yield only 

 virginoparae. Higher temeperatures oppose the action of short days, thus fa- 

 vouring the appearance of virginoparae. 



The effect of photoperiods of intermediate length is particularly instructive. 

 With a daily light period of 14^/2 hr. Megoura apterae often produce both 

 types of offspring; but serial collections of the progeny show that the parent 

 alternates as a virginopara-producer and an ovipara-producer. Now, if light 

 and temperature were acting directly on the embryos developing in the abdo- 

 men of the parent, we might expect that the two forms would be born in ran- 

 dom sequence. The fact that this does not occur suggests that the determin- 

 ation of the embryos is under maternal control. 



The nature of the switching mechanism is not kno\vn, although a humoral 

 basis is suspected. Experiments have indicated that the controlling system 

 begins to operate at, or shortly before, the birth of the mother, and it continues 



20 Syrapos. Kvolutio Insectonim 305 



