Apicomplexa

Question 1

What are some characteristics of apicomplexans?

Answer 1

• Obligate intracellular parasites of animals
• Type of protozoans (unicellular animals as opposed to metazoa) known as alveolates.
• Some cause important human (malaria, toxoplasmosis, cryptosporidiosis) and livestock diseases.

LIFE CYCLES
- complex, often two hosts
- Undergo specialised modes of cell division (endodyogeny, schizogony,endopolygeny)
- haploid genome throughout most of their life cycles

ORGANELLES
- set of apical organelles required for host cell invasion (“apical complex”; hence “Apicomplexa”).
- secondary plastid (apicoplast) acquired through secondary endosymbiosis)

Question 2

What distinguishing feature do alveolates have?

Answer 2

Alveoli = flattened vesicles underneath the plasma membrane. This is the inner membrane complex in apicomplexans.

Apicomplexans are one of 3 major groups of protists within alveolates.

Question 3

What is the apicoplast?

Answer 3

A plastid in apicomplexans.

Evolved via:
cyanobacterium endosymbiosis within red algae -> endosymbiosis of this within chromoalveolates -> animal parasitism.

4 membranes (2 chloroplast -> 1 red algae -> 1 host “chromoalveolate” membrane.

Contains:
35kb circular genome (~70 genes for rRNAs, tRNAs, transcritpion & translation factors) Regulating gene expression.
Loss of photosynthesis related genes.

Functions in metabolism:
- fatty acids (membrane lipids)
- isoprenoids (membrane lipids)
- Fe-S clusters (redox and electron transport reactions)

Question 4

Function and organelles of apical complex

Answer 4

Involved in host cell invasion.

Three main types of secretory apical organelles:
- rhoptries
- micronemes
- dense granules

Question 5

How do apicomplexans replicate? (Fig1)

Answer 5

Apicomplexan parasites divide by schizogony or endodyogeny:
- genome replication
- closed mitosis (nuclear division w/o breakdown of nuclear membrane)
- cell division by endodyogeny (internal budding; e.g. Toxoplasma) or schizogony (multiple daughter cells from syncitial mother cell; e.g Plasmodium)

As opposed to mammalian cells:
- genome replication
- open mitosis (breakdown of nuclear membrane, chromosome segregation,
re-formation of nuclear membrane)
- cell division (cytokinesis) by fission of the mother cell -> two daughter cells

Question 6

Name the two main differences between apicomplexan and mammalian cell division

Answer 6

1. Open (Api) vs. closed (Mam) mitosis
2. Cell division by endodyogeny/Schizogeny /endopolygeny (Api) vs. simple fission (mam)