Meiotic cytokinetic apparatus in the formation of the linear spore tetrads of Conocephalum japonicum (Bryophyta)

Most bryophytes produce tetrahedral spore tetrads. However, linear spore tetrads have been reported to occur in Conocephalum japonicum (Thunb.) Grolle. In this study, the distribution of microtubules (MTs) during meiosis in C. japonicum was examined to determine the division pattern resulting in a linear tetrad. Spore mother cells in the pre-meiotic stage were cylindrical with randomly distributed cytoplasmic MTs. In the prophase-metaphase transition, spindle MTs replaced cytoplasmic MTs and a barrel-shaped spindle with two flattened poles developed. Cortical MT arrays were not detectable throughout meiosis. Although a phragmoplast appeared between sister nuclei in telophase-I, it disappeared without expanding to the parental cell wall. Metaphase-II spindles oriented parallel to the long axis of the cell and in tandem to each other resulted in a linear arrangement of telophase nuclei. Radial arrays of MTs developed from the nuclear surfaces and three phragmoplasts appeared among the four nuclei to produce four spores. Two phragmoplasts separating the paired sister nuclei appeared prior to the appearance of a phragmoplast between non-sister nuclei. The MT cycle is basically the same as that reported in meiosis of C. conicum, which produces non-linear tetrads, A morphometric study indicated that the difference in the division pattern between C. conicum and C. japonicum is due to a difference in the shape of spore mother cells. The cylindrical shape of sporocytes of C. japonicum restricts the orientation of spindles and phragmoplasts so that the four resultant spores are arranged linearly.