EB14

Question 1

What are rotifers

Answer 1

a phylum of invertebtrates

more closely related to molluscs and flatworms than insects or nematodes

Question 2

what are the 4 rotifer taxa

Answer 2

1. monogononts
2. acanthocephala
3. seisonids
4. Bdelloids

Question 3

describe monogononts

Answer 3

• ~1600 spp.
• found in fresh water and marine, free living and commensal
• facultively sexual (heterogony)
• tend to be aseuxal in fresh water and sexual in marine

Question 4

describe acanthocephala

Answer 4

• ~1200 spp
• found in marine/freshwater*mostly. or terrestrial hosts
• obligate parasites of vertebrates and arthropods
• obligate sexual

Question 5

describe seisnoids

Answer 5

~5 spp.

* found in marine habitats, commensals or gills of crustaceans: obligately sexual.

Question 6

describe bdelloids

Answer 6

460 species
freshwater (few marine)
liminiterrestrial
free living or commensal
obligate ancient asexuals SCANDAL

Question 7

Describe bdelloid life cycle

Answer 7

adult -> apomictic egg -> hatch small copies of mother -> grows and produces her clone
Dessication: dormant stage in dry habitat

Question 8

Why is it good to study bdelloids

Answer 8

the exception that tests the rule

Question 9

Do Bdelloids have unusual mechanism to prevent accumulation of mutations

Answer 9

Experiment showed that infact bdelloids have an more inefficient purifying selection against deleterious mutations in mtDNA, with an excess of nonS relative to synon. compared to other clades like monogs.
*this indicates increase rate of assimialtion of deleterious mutations.
but this doesnt seem to be causing many problems for the rotifer. *could be unsuaul mechs preventing meltdown however. (rates gettin higher).
*for now assume mutations do accumualte without much effect on the population - therefore bdelloids dont support mutation as key to sex

Question 10

How can asexuals (bdelloids) benefit from lack of recombination

Answer 10

asexuals allele might be able to diverge and accumualte differetn beneficial mutations or evolve different functions which could double no. of indep evolving genes.
- e.g. LEA1 protein involve in dessication tolerance has two diverging alleles of different functions

Question 11

Differences in gene shufflign between asexual and sexuals

Answer 11

Sexual = independent segregation of chromosomes
Sexual  = recombination allow mix up of alelles (asexuals linked)
Sexual = recombiantion within loci homogenises where as asexuals alleles no longer licked no can potentiall diverge.

Question 12

Describe complexity of bdlloid genome

Answer 12

1. genome duplication = tetraploid
2. sets diverged (two sets of chromosome pairs), indep evolution.
3. further duplication events within chromosome pairs etc.
4. tetraploidy is degenerating.
5. collinearity between two sets degenerates.
6. despite prediction of divergence as in LEA1 alelles are nearly all in co-linear pairs.
   
   • alignment of 4 regions in P. roseola shows perfect collinearity within each set.
   • collinear pairs shown to be 96% identical.

Question 13

what is the effect of degnerate tetraploidy

Answer 13

gives two or more truly independent coppies of some genes however others are back down to one after gene loses, still doesnt fix problem of whole genome linkage, lots of asexuals are polyploid or have lots of gene duplications but havent persisted.

Question 14

what causes gene conversion

Answer 14

Desiccation leads to the break down of rotifer DNA by multiple ds breaks.
In order to repair the DNA, collinear pairs will use their sister allele as a template, leading to increased similarity between the two, this is known as gene conversion.
causes frequent gene conversion between them

Question 15

how could gene conversion help combat deleterious mutations

Answer 15

could replace bad alleles with good ones

*could be a way of dealing with mutation accumulation?

Question 16

does gene conversion provide something unique in rotifer asexuals

Answer 16

no as it brings copies together but doesnt change the fact that the whole genome is linked so would apply equally to any asexual

Question 17

Describe HGT in rotifer

Answer 17

During this DNA repair event foreign DNA can also be incorporated into the genome of the bdelloids and in some cases this can be expressed to produce functional proteins.
This horizontal gene transfer (HGT) has been hypothesized to be responsible for the bdelloids evolutionary success as asexuals, whereby they maintain a selective advantage by introducing favourable genes from other organisms, including genes from not only other animals but also plants, bacteria and fungi. In fact, 10% of genes expressed by bdelloids are of non-metazoan ancestry
* unclear how often this happens.
* provides a source of new potentially benefical genes

Question 18

do bdelloid rotifers do something like sex?

Answer 18

No two publications claimed to show evidence of genetic exchange in rotifers but they are mutally contradictory.

Question 19

testing RQ - hypothesis: bdelloids dont have parasites

Answer 19

*bdelloids attacked by 63 described species of parasitc fungi and water moulds

Question 20

testing RQ hypothesis: parasites not virulent

Answer 20

Harposporiu, Rotiferophthora = both very virulent.

Question 21

testing RQ hypothesis: parasites only recently started to attack them, no long standing coevolution

Answer 21

Some are more recent but for example Rotiferopthora is monophyletic and does not look like a recent pathogen - estimated to be 46.2 mill years conservatively. - bdelloids must be even older.
oldest fossil evidence of animal parasitsm support cretaceous diversification of fungi arthorpod symbiosis.
bdelloids have been associated with virulent parasites for ~50Ma etc.

Question 22

testing RQ hypothesis: the unusual ecology of bdelloids help limit extended exposure to parasites

Answer 22

IF host live in a structred metapopulation and hosts and parasites disperse more than parasites
THEN asexual hosts could outrun co-evolving parasite in space and time and their persistance could be explained by RQ.
** to meet this asumption hosts need to have an effectively parasite free life history phase in which they disperse more rapidly than parasites.

Question 23

Bdelloids parasite infection experiment

Answer 23

infected with Rotiferopthora spp.

• quickly exterminated when moist by lab cultures
• dried for increasing lengths of time, end of drought bought back stronger and rescued from parasites.
• after 5 weeks completely rescued from parasite in time.
• when combined with dispersal in wind chmber just 7 days wind dispersal dissectation dissociated fungus from host.

Question 24

Three ways to explain rotifer success

Answer 24

1. HGT : help bring genes from different back ground together - hill robertson hypothssi but only if homologous and rapid - not yet shown.
2. Gene conversion and tetraploidy: help deal with deleterious mutations by overwriting bad genes.
3. spatio temporal escape. in line with RQ

Question 25

based on RQ and spatio temporal escape how would we expect fungal parasitism of rotifers to change after a drought

Answer 25

fungal parasitism should be reduced after a drought

Question 26

what could we expect to see given spatio temporal escape of rotifers in nature.

Answer 26

1. parasites incidence should be lower in moss patches high up on trees than on ground (where they’ve been blown).

Question 27

how did rotifer parsitism change over different weather conditions

Answer 27

temp smame but different rainfall between months
used two time periods to test level of infection in rotifers at free heights.
in spring overall infection rate was low but there was a tend towards lower patches being more parasitized than expected.
after more rainfall more parasitised and again lower the rotifer the more parasites
*suggests rotifers meet the requirements for spatio temporal escape.