Development of abnormal strobili in Lycopodium annotinum as an example of the reversion phenomenon in lower vascular plants

Abstract

The formation of strobili, which are groups of sporangia-bearing sporophylls on shoot apices, terminates the axis growth in the majority of lycopods. Here, we present an interesting developmental aberration in Lycopodium annotinum L. The structure of abnormal strobili was histologically analyzed using the anatomical sections. Additionally, the expression patterns of LAMB1 and LAMB2, which are known to be specific to L. annotinum sporogenous and vegetative tissues, respectively, were comparatively analyzed in normal and abnormal strobili. Histological analyses revealed that the morphological and anatomical changes in abnormal strobili were related to the resumption of the vegetative growth of the strobilus apex after the production of several sporophylls and sporangia. The identity of both distinct zones of abnormal strobili, i.e., the basal sporangial part and the apical proliferative part, was confirmed by expression patterns of LAMB1 and LAMB2, indicating that microphylls in the apical proliferative zone were of the vegetative type. The results strongly suggest a change of meristem fate and activity in the process of abnormal strobilus formation. The abnormal strobili in L. annotinum could be a specific example of reversion in lower vascular plants. This phenomenon in lycopods is discussed in a developmental context.