Aberrant Ovules and Megagametophytes in Rhododendron nuttallii (Ericaceae)

Abstract

During study of megagametophyte development in Rhododendron nuttallii at daily intervals from anthesis through pollination 2 d later to 21 d after pollination using clearing techniques, 14 different deviations from the normal pattern of development were encountered. These are grouped into three general categories: aberrant ovule development, aberrant megagametophytes, and disintegrated(ing) megagametophytes. A fourth category, "overripe" megagametophytes, is also recognized. Abnormal ovules included nonanatropous ones, those showing asymmetric integument development with the nucellus not enclosed and "double" ovules. Unusual gametophyte development was seen in the functioning of a spore other than the chalazal one, functioning of more than one spore of the same tetrad, an extremely long nucellus with gametophytic tissue partly or wholly disintegrated, nuclei clustered in the center, nuclei of different sizes, small vacuolate cellular gametophyte, egg apparatus turned 90⚬-180⚬, and pollen tube overgrown within the gametophyte. Disintegration occurred at different developmental stages: near the tetrad stage, during gametophyte development from the functional spore but before enlargement, and, in pollinated ovaries, of mature gametophytes in ovules not entered by a pollen tube (rarely fertilized ones). Overripe gametophytes include some early stages, with extra large nuclei, that persist until late stages are prevalent, and mature gametophytes in unpollinated pistils at a time when fertilization would normally have occurred. A total of 33,688 ovules were tallied as to stage of development; through the 25 collection dates (including four unpollinated controls) an average of almost 8.7% (2,927) of these were classed as aberrant (overripe gametophytes not included). If prefertilization aberrancies alone are considered (eliminating disintegrating mature gametophytes), an average of just over 5.6% (1,900) were abnormal. Some aberrancies were more consistent in appearance or commoner than others: early disintegrating gametophytes occurred in all collections, and disintegration at maturity was rather common in the late pollinated collections. Since part of the study was directed toward determining if there was any effect of placental position on the stage of gametophyte development, such information was also available for aberrant ovules. Of the deviant types, five occurred too infrequently (only one to three ovules) to consider, three appeared to be randomly distributed, but five were, and one tended to be, associated with particular portions of the placenta, e.g., the gametophyte disintegrating at maturity. The scarcity of information about aberrant ovules and gametophytes and the relation of these to the commonly occurring low ratio of number of seeds set to number of ovules produced is discussed.