The quadripolar microtubule system in lower land plants |
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Authors: | Roy C Brown Betty E Lemmon |
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Institution: | (1) Department of Biology, University of Southwestern Louisiana, Lafayette, Louisiana, USA |
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Abstract: | The quadripolar microtubule system (QMS) is a complex array that is associated with predivision establishment of quadripolarity
in sporocytes of lower plants (bryophytes and lycopsids). The QMS unerringly predicts the polarity of the two meiotic divisions
and plays a central role in development of both the mitotic apparatus (MA) and cytokinetic apparatus (CA) which together accomplish
quadripartitioning of the sporocyte into four haploid spores. The QMS is typically, but not exclusively, associated with monoplastidy
and precocious quadrilobing of the cytoplasm. In early meiotic prophase the single plastid divides and the resultant plastids
migrate so that either the tips of two plastids or the four plastids resulting from a second division are located in the future
spore domains. Microtubules that emanate from the plastid tips or from individual plastids in the spore domains interact in
the future planes of cytokinesis and give rise to the QMS. The QMS, which encages the prophase nucleus, consists of at least
four and usually six (when spore domains are in tetrahedral arrangement) bipolar spindle-like arrays of microtubules presumably
with minus ends at plastids in spore domains and plus ends interacting in the future plane of cytokinesis. Each of the six
arrays is essentially like the single axial microtubule system (AMS) that intersects the division site and is transformed
into the spindle in monoplastidic mitosis in hornworts. As comparative data accumulate, it appears that the AMS is not unique
to monoplastidic cell division but instead represents a basic microtubule arrangement that survives as spindle and phragmoplast
in cell division of higher plants. |
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Keywords: | Cytokinesis Evolution Meiosis Microtubules Mitosis Sporogenesis |
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