HETEROCHRONY AND HETEROBLASTIC LEAF DEVELOPMENT IN TWO SUBSPECIES OF CUCURBITA ARGYROSPERMA (CUCURBITACEAE)

Abstract

To date, reports of paedomorphosis at the whole plant or shoot level have been loosely based on whole plant form or on the sequence of leaf shapes produced along the shoot (heteroblasty). However, interpreting the significance of heterochrony in the evolutionary loss or gain of heteroblasty based on mature leaf forms assumes that all leaves with the same shape arose through very similar modes of organogenesis. This study examines this assumption in two subspecies of Cucurbita argyrosperma, one that is wild and heteroblastic and a second that is cultivated and not markedly heteroblastic. All leaves of the cultivar are visually similar to early leaves of the wild subspecies. The cultivar is considered to be the progenitor of the wild subspecies. Scanning electron microscopy and allometry of developing leaves showed that at early nodal positions along the primary shoot, leaf development in both subspecies was similar. At later nodal positions, very young leaves of both subspecies were more similar to each other than to leaves at earlier nodal positions within the same plant at the same stage of development. This early similarity was masked in the mature shapes of later leaves due to subsequent differences in allometric growth. Thus a simple hypothesis of paedomorphosis in which the early leaf form in the progenitor is simply reiterated at later nodal positions in the cultivar is not supported by patterns of leaf development.