Polar organizers in monoplastidic mitosis of hepatics (Bryophyta)

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Abstract
Archesporial cells of the hepaticMonoclea gottscheiLindb. undergo a series of monoplastidic mitotic divisions prior to enlarging into sporocytes. Interphase cells have a nuclear‐based endoplasmic microtubule system that is predominantly aligned in the long axis of the cell and lack a hoop‐like cortical system. No preprophase bands (PPBs) are formed. Prior to mitosis, the single plastid divides and daughter plastids move to the incipient spindle poles as is typical of monoplastidic cell division. However, the plastids do not serve as microtubule organizing centers (MTOCs) as they do in both mitosis and meiosis of hornworts and lycopsids, and in meiosis of mosses. Rather, microtubules of the developing spindle emanate from distinct polar organizers (POs) arising just outside the nuclear envelope as in polyplastidic mitosis in other hepatics. The POs, which appear to arise de novo on opposite ends of the nucleus during preprophase, consist of vesicles, endoplasmic reticulum, and radiating microtubules. The developmental and evolutionary significance of distinct POs, plastid MTOCs, and the diffuse MTOCs of higher plants is discussed.