Evolution Of Eukaryotic Algae PDF

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Evolution Of Eukaryotic AlgaeAdditional Notes Date Links Week Week 8The evolution of the first photosynthetic eukaryoteThe chloroplast organelle arose from a cyanobacterium ~ 1 billion years ago.A free-living cyano. was engulfed by a feeding amoeba-like eukaryoteRather than being digested, the cyano...

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Evolution Of Eukaryotic Algae

Week 8 The evolution of the first photosynthetic eukaryote The chloroplast organelle arose from a cyanobacterium ~ 1 billion years ago. A free-living cyano. was engulfed by a feeding amoeba-like eukaryote Rather than being digested, the cyano became an endosymbiont – providing fixed carbon and oxygen to its host in return for a safe, nutrient-rich niche Over evolutionary time, there was a transition for facultative symbiont to obligate symbiont to organelle facultative symbiont meaning that it still had the choice of becoming a free living bacteria obligate meaning it now relies completely on its host cell for survival now there is a cyanobacteria that is within a host cell, but this cyanobacteria doesnt know it's been engulfed so it continues to photosynthesise (amoeba is translucent so it can still see sunlight) so it continues to excrete O2 and sugars Cyanobacteria live in fixed communities where an important strategy is to trade nutrients with other bacteria they do this to attract heterotrophic bacteria who themselves excrete vitamins for the cyanobacteria to exploit Why does the bacteria not get digested? a way that this could have happened was a mutation either in the host cell that affected its lytic enzymes so it couldn't break down the cyanobacteria wall or in the cyanobacteria itself that somehow increased the strength of its wall now this means that the bacteria is able to persist for a longer period of time within the host - and will carry on growing using the nutrients within, and carry on photosynthesising and dividing

now the rate of cell division of the cyanobacteria is higher than the rate of its lysis by the host bacteria under these conditions the initially captured cyanobacteria can now replicate - populating the eukaryotic cell

The process of transitioning from a free living bacteria to a chloroplast involves a lot of gene loss this is due to selective pressures on the cyanobacteria genome (symbiont gene): The free-living cyanobacterial ancestor had several thousand genes. Genes no longer required for endosymbiotic existence (e.g. genes for flagella, cell wall, scavenging micronutrients) were quickly lost. this due to streamlining its genome to allow for faster replication Genes for metabolic pathways duplicated by host were lost Many genes were transferred from cyano. to host nucleus. - this took place due to many of the early cyanobacteria that were engulfed were lysed and therefore there was spillage of their DNA into the cytoplasm, which can make its way to the host nucleus - this constant bombardment of bacterial DNA on the nucleus over evolutionary time allowed for nuclear copies to be made of this DNA - now this allows for this DNA to be lost in the cyanobacteria as it is now part of the host cell Algal family tree depicted in the slide is the endosymbiotic event of chloroplasts and host cells and their branching into the 3 main types of algae Chlorophytes (green algae): these have evolved all terrestrial plants have single celled representatives as well as multi have Chl. a (chlorophyll a) and Chl. b Rhodophytes (red algae): have single celled representatives as well as multi have Chl a and phycobilins Glaucocystophyta: an group that separated early

no examples of multicellular organisms here have Chl a and phycobilins (slightly diff wavelength absorption to red algae) originally thought that these 3 arose from 3 separate endosymbiotic events but it is now clear from molecular studies that they likely all arose from the same endosymbiotic event and that the original cyanobacteria contained all 3 pigments of Chl a, Chl b and phycobilins and Chl b was lost in red algae and glauco and phycobilins were lost in green algae The chlorophyte lineage gave rise to all the land plants the land plants can all be traced back to primitive green algae clearly at some stage there has been evolution to give multicellularity to give rise to multicellular plants evidence for this can be seen in the green algae red arrow points to free living algae (at the top) which are closely related to algae which forms clusters - you can see here the beginnings of multicellularity the 3rd picture then shows differentiation of cells into different cell types

glaucocystophytes interesting as their chloroplast has retained the peptidoglycan wall of the og gram negative cyanobacterium this finding was significant in proving that cyanobacteria gave rise to the chloroplast

further endosymbiotic events are able to occur, this is represented by the labels of 'secondary' and 'tertiary' in the slide

Further endosymbiotic events the original eukaryote that engulfed the cyanobacteria itself can be engulfed by other eukarya this allows other eukarya to make the same transition of going from feeding to harnessing energy from sunlight using photosynthesis once captured the only truly important part is the chloroplast so over evolutionary time the rest of the original photosynthetic eukaryote is reduced into its organelle so over time other components e.g. mitochondria and ER are lost but remember that the nucleus contains essential genes which were transferred into it from the original cyanobacteria so before this is lost a similar gene transfer is made to the new host nucleus before the original nucleus can be lost there are instances where this transfer of genes from old to new nucleus is still incomplete, this constitutes the groups known as the chlorarachniophytes and cryptophytes each with green and red algae engulfed respectively chlorarachniophytes and cryptophytes have 4 genomes

nuclear, nucleomorph, chloroplast and mitochondria there has been a strong selective pressure to compact the chromosomes within these genomes there are 3 chromosomes as this is the minimum number required for assembly into the mitotic spindle separation during mitosis so chromosome number reduced to the point where it can no longer be reduced known as the bonsai chromosomes

Euglena obtained their chloroplast in a separate ‘green alga’ endosymbiosis in which the protozoan host was closely related to modern-day trypanosomes The remaining algal groups acquired their chloroplasts by secondary endosymbiosis involving red algae e.g. the heterokonts, haptophytes(e.g. Emiliania, Prymnesium) and apicomplexa The apicomplexa have retained a non-pigmented plastid with a reduced genome (potential drug target) finally, dinoflagellates: All dinoflagellates probably began with a chloroplast from a red alga. However, 50% of all species have since discarded their chloroplast (returned to being heterotrophs) Others replaced this with one from a green alga or a haptophyte (a case of tertiary endosymbiosis!) tertiary endosymbiosis as the haptophyto got its chloroplast from red alga, which in turn got its own chloroplast from the cyanobacteria Others have temporary chloroplasts (klepto plastids) obtained from their algal prey, and maintained for a few months without replication...