Algae

Question 1

When phagocytosis went wrong: the evolution of the first photosynthetic eukaryote

Answer 1

• Molecular analysis of chloroplast genes in plants and algae indicate that the organelle arose from a cyanobacterium ~ 1 billion years ago

Question 2

The endosymbiotic theory

Answer 2

• A free-living cyano, was engulfed by a feeding amoeba-like eukaryote
• rather than being digested the cyano became endo-symbiont - 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

Question 3

The selective pressures on the symbiont genome

Answer 3

• 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
• genes for metabolic pathways duplicated by host were lost
• many genes were transferred from cyano, to host nucleus

Question 4

The glaucocystophytes (glaucophytes)

Answer 4

• relatively insignificant group of freshwater alagae
• only ~13 species described
• however, interesting from an evolutionary perspective since the chloroplast has retained the peptidoglycan cell wall of the original Gram negative cyanobacterium

Question 5

Secondary endosymbiosis

Answer 5

Acquiring a second hand chloroplast

Question 6

The “Russian doll” - a eukaryote within a eukaryote

Answer 6

1. Capture of a photosynthetic eukaryote
2. Establishment of a symbiont
3. Symbiont reduced to an organelle
4. Nucleomorph lost

Question 7

Two algal groups are remarkable because

Answer 7

1. The cells possess FOUR genomes,
   each with a different evolutionary history:
   - A nuclear genome from the eukaryotic
   host
   - A nucleomorph genome from the
   eukaryotic alga
   - A chloroplast genome from a
   cyanobacterium
   - A mitochondrial genome from an -
   proteobacterium
2. Despite having different origins (red
   and green algae), the nucleomorph
   genomes show striking convergent
   evolution.
   Each has become miniaturized to form
   three tiny chromosomes, with a few
   hundred genes tightly packed so that
   there is very little intergenic space.

Question 8

Euglena

Answer 8

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 endosymbioses
involving red algae

Question 9

apicomplexa

Answer 9

The apicomplexa have retained a non-pigmented plastid (= apicoplast) with a simple genetic system

The apicoplast could be a drug target.

• Various antibiotics that target bacterial RNA polymerase or ribosomes are effective against some apicomplexan species

Question 10

The weird and wonderful world of the dinoflagellates

Answer 10

• 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!).
• Others have temporary chloroplasts
  (kleptoplastids) obtained from their algal prey, and maintained for a few months without replication

Question 11

Sea slugs

Answer 11

Another beautiful example of kleptoplastidy

Question 12

Elysia chlorotica

Answer 12

E.chlorotica can survive on sunlight and CO2
for their whole adult life (~9 months).

The chloroplasts do not divide and are not
transmitted to the eggs. Rather the juveniles
feed on Vaucheria to acquire new chloroplasts