Morphogenetic potential of Fraxinus ornus under the influence of the gall mite Aceria fraxinivora

Abstract

Under the influence of the mite Aceria (Eriophyes) fraxinivora Nal., Fraxinus ornus L. produces galls which are transformed buds ("klunkerns") in the vegetative and reproductive region. In the vegetative region, to which this study is restricted, the following changes occur after infection by the mite: (i) modification and eventual inhibition of the growth of the main shoot apex; (ii) inhibition of internodal expansion; (iii) inhibition and deformation of leaf growth; (iv) increase in the diameter of the axis frequently coupled with fasciations; (v) development of supernumerary branches; and (vi) formation of a great number and variety of appendages on the leaves and stems including the shoot apex. This study focusses on the development of these appendages. Many of them develop like little leaves, others like determinate stems, or even rudimentary shoots, and still others like emergences. Intermediates between these types of appendages occur including intermediates between emergences and trichomes. The appendages may be simple or branched, nonvascularized or more or less vascularized, irregularly grouped or arranged in a more or less phyllotactic pattern, leaf or stem borne, and separate from each other or more or less continuous with each other. Much of the enormous variation is continuous. Ten continua are distinguished according to 10 different criteria. The galls are compared with galls of other taxa, which consist of a similar range of appendages, and with naturally occurring continua between morphological categories. It is concluded that under the influence of the mite Aceria fraxinivora, Fraxinus ornus expresses a morphogenetic potential to form a continuum between morphological categories which under normal circumstances is not manifested.