ORGANELLE DISTRIBUTION AND APPORTIONMENT DURING MEIOSIS IN THE MICROSPOROCYTE OF GINKGO BILOBA L.

Microsporocytes of Ginkgo biloba were observed in stages of meiosis with both the light and electron microscopes. After telophase I, all of the plastids and mitochondria were located in the equatorial region of the cell and this non-random pattern of distribution was maintained until nuclear division had ceased. The ensuing apportionment of organelles among tetrad members was approximately equal, and was coincident with centripetal formation of the partitioning walls. Although no mechanisms for the initial equatorial positioning or later dispersal of the organelles could be ascertained, membrane proliferations were prevalent and may have played a role in maintaining the pattern of organelle distribution through the duration of the second division. Microtubules, which permeate the central region of the cell may have also functioned to maintain the non-random pattern of organelle distribution. THE MEIOTIC DIVISION leading to pollen formation in Ginkgo biloba was associated with an ordered distribution of organelles parallel to and confluent with the incipient cell plate of the first division. After telophase I, all of the plastids and mitochondria became centrally positioned and maintained this band-like distribution pattern until simultaneous partitioning of the tetrad was initiated. Mann (1924) first described this in G. biloba and proposed that chromosomal movement towards the cell poles generated a force which pulled the organelles into the equatorial region from their original peripheral positions. Vasil and Aldrich (1970) observed a similar pattern of organelle distribution during meiosis in microsporocytes of Podocarpus macrophyllus, but did not speculate on its formation or significance. Microscopic observations made during this study of G. biloba microsporocytes failed to yield additional information concerning the initial mechanism of organelle movement into the equatorial region of the cell or the final mechanism of organelle dispersion among the cells of the tetrad. However, these observations did confirm the presence of a complex network of microtubules and ER proliferations which appeared to stabilize the non-random pattern of distribution of plastids and mitochondria during the second nuclear division. The purpose of this paper is to demonstrate the ultrastructural network of microtubules and membrane elaborations in the central portion of the microsporocyte of G. biloba during meiosis, and to suggest that the role of this network is to, main'Received for publication 4 February 1975. 2 Present address: Department of Botany, University of California, Berkeley, 94720. I am particularly grateful to Dr. Zane B. Carothers for his help and guidance during the undertaking of this study. tain the integrity of the organelle distribution pattern as the division progresses. MATERIALS AND METHODs-Microsporangiate cones of G. biloba were collected at intervals from the University of Illinois campus during the spring months of 1973 and 1974. In March, 1974, branches bearing male cones were cut from the tree and placed into distilled water in the laboratory. There was no discernable difference on either gross or fine structural levels between "cultured" cones and those taken fresh from the tree. In an attempt to correlate gross morphological characters with developmental stages, cones and individual sporangia were measured on a systematic basis immediately after excision from the parent organ to which they were attached. Light microscopy-Freshly excised microsporangia were squashed on glass microscope slides and the microsporocytes were either stained with acetocarmine for chromatin determinations (Jensen, 1962), or stained with I2KI for starch (Jensen, 1962). Other material fixed for electron microscopy (see below) was sectioned at a thickness of l m and stained with either crystal violet or I2KI. Photomicrographs were taken with a Zeiss Phase Contrast Microscope. Electron microscopy-Microsporangia, as above, were bisected transversely and fixed in 2.5 % glutaraldehyde in 0.05 M phosphate buffer (pH 7.2) for 2-16 hr at 25 C. A 4-hr buffer wash was followed by post-fixation in 1% OSO4 in the phosphate buffer for 2 hr at room temperature. The tissue was then dehydrated in a graded ethanol series and embedded in Epon-812. Thin sections were stained with uranyl acetate and lead citrate