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Lecture 16

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Western University
Biology 1001A
Tom Haffie

Likelihood that modern multi-cellular life forms are monophyletic. - ● Not very likely at all; evidence shows that multicellularity has evolved multiple times over the course of evolution ● Many branches on the tree of life (specifically eukarya) where some organisms are all unicellular, some are all multicellular, and others where species can be both Characteristics of Volvocine algae that make them a useful model system for studying the transition to multicellularity. - ● Volvocine algae are a good model for multicellularity because they have both multicellular and unicellular species and have some who are intermediates ● Also, the divergence of these species (chlamydomonas and volvox specifically) is very recent, occurring only 50 million years ago compared to the 1200 million years ago divergence between them and animals or land plants Relative structure/function of Chlamydomonas vs. Volvox cells. - ● Chlamydomonas is eukaryotic unicellular organism, has a single chloroplast, an eyespot, a couple mitochondria and flagella ○ it asexually divides and when it does, it loses its flagella ● Volvox consists of a few thousand somatic cells and a handful of reproductive cells ○ volvox looks is transparent and looks like nothing is in there but the ECM (extracellular matrix) is holding it together; it is full of ECM, not hollow General process of Volvox asexual reproduction; role of somatic vs gonidial cells. - ● You start off with a single gonidial cell which divides multiple times ● After a small number of divisions, asymmetric division begins to occur and you start to see many small cells and some large cells forming ● After a bit, the large cells stop dividing but the small cells continue to divide ● Eventually, you end up with a handful of large cells on the outside, which are the gonidial cells, and a ton of small cells with their flagella facing inwards, which are the somatic cells; it is inside out! ● The whole cell inverts itself and this result sin the large gonidial cells being on the inside and the small somatic cells’flagella facing outwards now ● The cell then gets bigger and bigger by accumulating ECM ● Then, the gonidial cells inside break through the cell and go onto form their own volvox cells; as a result all of the somatic cells die Genetic approaches to identifying genes relevant in rise/maintenance of multicellularity. - ● Compare regulation of different genes ● Comparative genomic study and compare with unicellular organisms’genes ● Study volvox mutants and see which genes af
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