Evidence for partial homology: Examining the morphological and molecular relationships between shoot and leaf development in watermilfoil (Myriophyllum aquaticum)

There is evidence from both morphological and molecular studies that many shoots and leaves have analogous developmental characteristics which transcend their classical categorization. An example of this can be found in the expression patterns of the KNOX1 family of homeobox genes which function in maintenance of indeterminate growth and are expressed in both dissected leaves and shoots. Shoot and leaf morphological parallels were characterized in the aquatic angiosperm Myriophyllum aquaticum (Vel.) using a combination of scanning electron microscopy and standard light microscopy. A KNOX1 gene fragment was also sequenced from M. aquaticum and its pattern of expression was mapped at the shoot tip using RNA in situ hybridization as the primary technique. Leaves were found to develop lobes in an alternating basipetal fashion and originated from distinct generative centers at the leaf base. Within the tissues of the developing shoot, KNOX1 expression was found to be localized to the developing stem, provascular strands, dermal tissues of internodes, and between developing leaf bases. KNOX1 expression was also found within developing leaves where patterns varied depending on the age of the primordium. In leaves between plastochrones 1 and 3, KNOX1 is evenly expressed throughout the primordium. In older plastochrones, expression becomes localized the more recently developed lobes. By plastochrone 9, expression signal is no longer visible. The presence of distinct lobe forming centers at the base of the leaf and the corresponding KNOX1 expression during leaf and shoot development is indicative of developmental parallels between traditionally non homologous structures.