emes 446 FINAL review

hologenome theory of evolution

host and its symbionts work together and their genomes evolve together as a single unit

• benefit- microorganisms evolve faster which can help host

holobiont of coral

host & all of its microorganism/symbionts

hermatypic corals

HOST

2 types

1. massive- mound shaped, grow slowly (approx. 1 cm per year) ex. siderastrea sp.

2. branching- tree-like branch, or elkhorn shapes, grows rapidly (approx. 10 cm per year) ex. acropora sp.

• corals can reproduce asexually through fragmentation

• during sexual reproduction, the eggs typically do not have symbionts so they must pock them up from environment

• transmission not fully understood

• eukaryotes

anatomy of hermatypic corals

• polyp imbedded in “corallite”

• polyps within a colony are “clones;” arise from single polyp that divides asexually

• connected by layer of tissue

• feed on zooplankton/particles but…symbiotic algae in gastrodermis

reproduction and mass spawning

• corals are simultaneously hermaphroditic or occur as separate sexes

gamete production by 2 modes:

1. internal fertilization (brooding) followed by release of planula larvae

2. broadcast-spawning of gametes, followed by formation of planula larvae that are planktonic

asexual reprod. by division of polyps results in large areas of reefs being covered within a single genotype

• most corals spawn gametes in the water column in synchronized multispecies mass spawning

why coordinates spawning?

• increase fertilization rate

• overwhelm predators

• coordinate based on environment factors- spawning is during slack neap tides at times of low wave action (minimize spread of gametes by waves and turbulence= increase fert. rate and reduce damage to early coral embryos)

• hybrids b/w species

endolith

calcium carbonate skeleton in coral

microbial community within coral host

diverse microorganisms found in endolith, intracellular within tissue, in digestive tract, but MOST recovered from surface mucus layer

zooxanthellae

THE SYMBIONT

• dinoflagellate

• genus Symbiodinium

• looks more ball shaped in host

• photosynthetic

coral-zooxanthellae symbiosis

• coral provides CO2, nutrients

• alga fixes CO2 to carbohydrates (provide corals up to 90% of metabolic requirements)

• intracellular symbiont (found inside symbiosome in coral gastrodermis)

• many clades of symbiodinium (previously thought just one species but nope!)

zooxanthellae and their hosts

• clade C and clade A seem to be generalists (found in many hosts)

• clade G seems to be specialist (found in just 1 host)

when mapping biogeography of symbionts…

• clades A and B occurred in light-exposed tops

• clade C mostly in shaded sides or deeper water habitats

• suggests there are high-light and low-light adapted clades

symbiodinium

• symbiosis allows holobiont to thrive in nutrient poor water

• zooxanthellae and coral relationship often referred to as “keystone symbiosis”

• bleaching occurs when corals lose color due to loss of symbiotic zooxanthellae or their pigment

• corals can expel dinoflagellate symbioses and acquire new strains/clades

corals have thousands of microbes- many other symbionts just unsure what to do

adaptive bleaching hypothesis

when the environment changes, the loss of resident zooxanthellae is followed by establishment of a

“new” symbiotic interaction with different zooxanthellae that are better adapted to the new conditions

• ex. Some Symbiodidium clades seem to be more thermotolerant

• symbiotic flexibility allows for corals to swap symbiont clades that may allow the holobiont to respond to environmental stress