Excavates and SAR

How did algae belonging to the Excavate and SAR supergroups acquire plastids?

Via secondary or tertiary endosymbioses

What are the diferences between plastids that evolved from primary and secondary endosymbioses?

• A few organisms have secondary plastids that harbour a nucleomorph

• In other cases, the endosymbiont nucleus has been lost altogether

• Secondary plastids are characterized by additional membranes surrounding the plastid

  • Rather than the 2 membranes bounding a primary plastid, secondary plastids are surrounded by 4 or sometimes 3 membranes

How do plastid proteins encoded in the host nucleus make their way through the extra plastid membranes?

• Many algae with secondary plastids use the secretory system of the host cell to deliver proteins through the outer membrane of the plastid

• The TIC TOC translocons present in the inner 2 plastid membranes mediate transport of proteins across these membranes

What is a nucleomorph?

A highly reduced remnant nucleus from the endosymbiont

What are euglenoids?

The only algae in the Excavate supergroup

What are Excavates?

• Most are single-celled organisms with a variety of nutritional modes

• Group including heterotrophs, photosynthetic species, and parasites

What are Euglenophytes?

• The only Excavates known to have photosynthetic members

• Phylum that includes single-celled flagellates with very different modes of nutrition

List 4 modes of Euglenophyte nutrition.

• Predators that prey on other single-celled organisms

• Parasites, including some that infect humans

• Osmotrophs that absorb nutrients directly from their environments

• Photoautotrophs capable of photosynthesis

How are photosynthetic euglenoids hypothesized to have acquired their plastids?

Secondary endosymbiosis

1. It is thought that a green alga was ingested by an ancestor in the Excavate supergroup

2. The green alga nucleus and other cellular organelles except for the chloroplast were lost as the endosymbiont evolved into a plastid

3. Photosynthetic euglenoids have plastids that lack a nucleomorph and are surrounded by 3 membranes

What evidence supports the hypothesis that photosynthetic euglenoids acquired their plastids from a green algal endosymbiont?

1. Chloroplasts in Euglena are bounded by 3 membranes, a feature that is indicative of evolution from secondary endosymbiosis. At first, these plastids would have been bounded by 4 membranes. Apparently one was lost during evolution

2. Pigments used to absorb light during photosynthesis in Euglenoid plastids are similar to those in green algae

   1. Both have chlorophylls a and b

   2. Red algae have chlorophylls a and d

   3. Brown algae have chlorophylls a and c

3. Analysis of chloroplast DNA sequences revealed characteristics that were shared between a specific clade of green algae (Pyramimonas) and Euglena

   1. These characteristics were not found in other clades of green algae

List the 3 groups of organisms in the Euglenophyta clade.

1. Euglena

2. Kinetoplastida

3. Diplonomida

List 2 hypotheses for what could have happened during evolution to produce the mixture of photosynthetic and non-photosynthetic organisms belonging to the Euglenophyta today.

1. A photosynthetic ancestor gave rise to all the species that exist today. Non-photosynthetic organisms would have lost the ability to photosynthesize as they evolved

2. Plastids were acquired only in the lineage that gave rise to Euglena. The Kinetoplastids and Diplonomida would have never had photosynthetic abilities

What is Giardia lamblia?

• A Diplonomad that causes giardiasis

• Can be contracted by drinking untreated water from lakes, streams, or rivers

• Commonly contracted in BC

What is giardiasis?

An intestinal infection caused by Giardia lamblia

What are trypanosomes?

Kinetoplastids that cause African sleeping sickness

Can we find evidence of a photosynthetic past in the Kinetoplastid or the Diplonomida lineage?

1. Remnant organelles that could have at one time been a chloroplast have not been found in either lineage

2. The genomes of some Diplnomids and Kinetoplastids have been sequenced. These studies revealed that Kinetoplastids have several genes in their nucleus that encode homologs of Calvin Cycle enzymes. Trypanosomes use these proteins to synthesize carbohydrates in the absence of photosynthesis. Trypanosomes could have inherited these genes from an ancestor that had a plastid that was lost during evolution. If so, then photosynthetic abilities could have been acquired by an older ancestor that existed before Euglena split from the other 2 groups. This would have been followed by plastid loss in the other 2 groups.

What is a kinetoplast?

• A distinct subcellular structure inside the mitochondria of Kinetoplastids

• Not related to the chloroplast

Which algal lineages have plastids derived from a red alga?

• Hacrobia

• Brown algae

• Diatoms

• Apicomplexans

• Dinoflagellates

Which algal clade has species that acquired plastids separately, either from a cyanobacterium or a green alga?

Rhizaria

List 2 hypotheses for where in evolutionary history plastids were acquired by organisms in the SAR lineage.

1. A single engulfment event occurred early in the evolution of this lineage, followed by loss in some lineages

2. There were multiple different engulfment events that occurred separately in different lineages

The known Rhizarian algae belong to which phylum and clades?

• Phylum Cercozoa

• Chlorarachniophytes and Silicofilosea clades

What are the Chlorarachniophytes?

A group of unicellular, photosynthetic marine organisms

How did the Chlorarachniophytes acquire their plastids?

Via evolution of a secondary endosymbiosis

What is Paulinella chromatophora?

• An alga belonging to the Silicofilosea

• Has a plastid of primary origins that is in an earlier stage of evolution than the algae in the Plantae lineage

What evidence supports the hypothesis that Chlorarachniophyte plastids evolved from a secondary endosymbiosis?

1. Chlorarachniophytes are one of 2 SAR groups in which the primary algal nucleus has not been completely lost. A remnant nucleomorph still exists. The presence of a nucleomorph is the “smoking gun” that indicates the plastid arose from a eukaryotic cell.

2. There are 4 membranes surrounding the chloroplast. The extra membranes characterize plastids that evolved from secondary endosymbiosis.

What are the only known algae to have species with nucleomorph-containing plastids?

• Cryptophytes (Hacrobia)

• Chlorarachniophytes (Cercozoa)