Microsporogenesis of Cycas and its systematic implications

Abstract

Taxonomists are divided over the infrageneric classification and species delimitation within the genus Cycas. The division is largely determined by whether a broad or narrow species concept is adopted, the latter approach being based on apparently minor morphological differences. It is well known that cytokinesis in the cells of pollen provides important evidence for plant taxonomy, particularly at the higher taxonomic level. Here we present the first broad comparison of the cytokinesis of male meiosis in five species of Cycas. A comparative analysis of microsporogenesis in Cycas was carried out using conventional microscopy, semi‐thin sectioning, histochemistry, and fluorescence microscopy with a focus on the cytokinesis of meiosis in the pollen of dividing cells. Our observations confirmed that, contrary to previous reports, the cytokinesis in male meiosis of five species in Cycas is simultaneous at the end of second meiosis. The basic model of microsporogenesis and its systematic implications in Cycas is discussed based both on previous reports and our new results.

Spatiotemporal features of microsporogenesis in the cycad species Macrozamia communis

“…ACWIs at a future cytokinetic site in microsporogenesis are thus likely a general phenomenon in cycads. It is noted here that microsporogenic cytokinesis in cycads has been described as either simultaneous (Singh, 1978; Audran, 1981; Zhang et al, 2012), successive (Baird, 1939; De Silva and Tambiah, 1952; Rao, 1961; Wu et al, 1995), or mixed (Ouyang et al, 2004) based on limited light microscopy studies. Based on this study and reevaluation of the literature, we argue that the cytokinesis type in microsporogenesis in cycads is most closely related to the intermediate type as exemplified by Magnolia .…”

“…communis occurs in two steps, first at the cell periphery, and then, after a significant delay, in the cell interior. Even though intermediate cytokinetic structures in the cell interior were not examined in previous studies of several other cycad species, a localized cell‐wall‐thickening region corresponding to the callose ring and a future cytokinetic site was observed during the period between the first and second nuclear divisions (Singh, 1978; Audran, 1981; Zhang et al, 2012). ACWIs at a future cytokinetic site in microsporogenesis are thus likely a general phenomenon in cycads.…”

“…Callose deposition is a common event during meiosis in plants (Singh, 1978; Johri, 1984; Bhatia and Malik, 1996; Lü et al, 2003; Lu et al, 2011; Zhang et al, 2012). In M. communis , two callose rings form at the sites where future cell plates will join the parental cell wall in advance of the mini‐phragmoplast‐based process of cell plate formation in the interior of the cell.…”

“…Semithin sections (1–2 µm thick) were cut using either a Reichert‐Jung or a Leica RM 2155 microtome. Sections were stained with either 0.05% w/v toluidine blue O (TBO), periodic acid–Schiff's reagent and 0.25% haematoxylin, or 1 µg/mL 4′,6‐diamidino‐2‐phenylindole (DAPI) and 0.005% aniline blue (Zhang et al, 2012). Sections were examined and photographed using a Leica DMLB light microscope for both bright‐field microscopy and epifluorescence microscopy with the UV channel.…”

“…For a comparison, either simultaneous or successive cytokinesis was reported for microsporogenesis in gymnosperm species other than cycads (Maheshwari, 1935; Moitra and Bhatnagar, 1982; Anderson and Owens, 2000; Lü et al, 2003; Zhang et al, 2008; Lu et al, 2011). Currently, only limited information is available on the process of microsporogenesis in several cycads, but the cytokinetic process seems to be similar to the intermediate type (Singh, 1978; Audran, 1981; Zhang et al, 2012).…”