EEB340 Lecture 14.docx

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10 Apr 2012
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Lecture 14: The Evolution of Alternation of Generations
Alternation of Generations
Sporogenesis
Zygotic Meiosis in Coleochata/Chara
Reproduction in Hepatophyta/Anthoceraphyta/Bryophyta
- sporogenous cells beign to prepare to undergo meiosis
o all connected by plasmodesmata
o each have a callose wall of beta 1-3 glucan, which is also in the sieve plates of
phloem
o the cells are eventually separated from one another by the glucan, which
makes them symplastically isolated
- spore mother cell/sporocyte undergoes meiosis to give rise to four haploid
spores
o forms trilete spores (basal condition)
- callose wall between each of the four spores symplastiaclly isolated
- degradation of the callose wall now each spore begins to develop and deposit
sporopollenin
- callose wall then degrades and tapetum beings to synthesize and secrete
sporopollening, making spore cell walls more complex
- end up with mature spores with sporopollening in their walls
Chara/Coleochaetes
- zygotic meiosis
- believed that to go from zygotic to sporic meiosis life cycle, there was a delay in
meiosis and interrupted by mitotic divisions of zygotic
- egg is retained on the haploid individual
o feature was co-opted by land plants
- transfer cell walls are important for short distance transport because the zygote is
symplastically isolated
- even adjacent filaments will provide nutrients for the zygote
- this feature was co-opted by land plants
- zygote dependent initially on the gametophyte for survival
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