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Vector-Parasite Interactions > vector biology & genetics > Flashcards

vector biology & genetics Flashcards

1
Q

Anopheles distribution

A
  • tip of africa to northern europe
  • doesn’t match malaria distribution
    • some species better at transmitting
  • temperature and vector suitability determines malaria
    • much higher in sub-saharan africa
  • some suitable areas have no malaria
    • mainly through prevention and healthcare
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2
Q

EIR

A
  • entomological inoculation rate
  • how many infectious bites individual receives per unit time
  • areas of high EIR
    • hundreds of bites a season, each bite has 20-30% chance of transmission
  • overlaps with Anopheles gambiae distribution
    • A. gambiae - 90% of malaria-related deaths
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3
Q

mosquitoes as vectors

A
  • anthropophagic
  • high parasite susceptibility
  • lifespan long enough for parasite life cycle
  • density and habitat preference
    • ensures vertebrate host contact
  • frequent blood feeding
  • endophagic and endophilic
    • eating and resting
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4
Q

genetic approaches to studying vector biology

A
  • chromosomal view
  • classical genetic view
  • genomics view
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5
Q

species complex

A
  • group of closely related species that fulfill the definition of a species, but are morphologically identical
  • vary in:
    • spatial or temporal association with each other
    • physiological behaviour
    • vector competence
    • chromosomal and molecular markers
      • used to study mating and gene flow
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6
Q

A. gambiae complex

A
  • 6 species in complex
  • occupy different regions but are sometimes sympatric
  • ss - sensu stricto
  • sl - sensu lato
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7
Q

biological species concept

A
  • offspring of two separate species that have interbred will be sterile
  • e.g. A. gambiae and A. arabiensis
    • species in the complex fulfill this definition
      • reproductively isolated groups
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8
Q

chromosomal forms

A
  • refers to particular class of chromosomes in mosquito cells that have high energy requirements and turnover
    • salivary glands, ovaries, nerve cells
  • variations in these chromosomes
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9
Q

mosquito chromosomes

A
  • 3 homologous chromosomes
  • includes 1 sex chromosome
  • cells with chromosomal forms have multiple rounds of replication without cell division
    • forms polytene chromosomes
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10
Q

polytene chromosomes

A
  • multiple copies of chromosomes align to form one huge chromosome
  • observed under light microscope
    • characteristic banding pattern
    • light and dark
    • highly specific and reproducible
    • corresponds to gene order
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11
Q

chromosome inversions

A
  • easy to identify with polytene chromosomes
    • genetic markers in A. gambiae populations
  • pericentric inversion
    • includes centromere
  • paracentric inversion
    • doesn’t include cetnromere
  • inversion requires 2 chromosome breaks and 180 degree rotation
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12
Q

heterozygous inversion

A
  • only 1 chromosome inverted
  • lack of homology in inverted region
  • can’t pair up properly
  • inverted region loops out so homologous regions can bind
    • characteristic markers
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13
Q

frequency of inversions

A
  • varies
  • at least 5 types known
  • some have been assoicated with adaptation to new ecological niches
  • can use to study gene flow among populations
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14
Q

inversion frequency in Mali

A
  • sample transects of whole country
  • 3 groups found upon genotyping
    • different combinations of inversions
  • Savannah group
    • similar based on inversion class
    • rain dependent larval sites
  • Mopti group
    • semi-permanent sites, humidity and water
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15
Q

effect of inversions

A
  • no loss/gain of genetic material
  • generally no phenotypic effect
  • in heterozygotes
    • reproductive consequences
    • due to homologue pairing at meiosis
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16
Q

inversions and meiosis

A
  • usually homologous chromosomes align and crossover occurs
  • crossover can occur within inverted region
  • loss of genetic information
    • one dicentric gamete (2 centromeres)
    • one acentric gamete
    • neither are viable
  • viable gametes only when no crossover
  • also crossover suppression
    • occurs at a reduced rate
17
Q

allele combinations at inversion genes

A
  • very tight linkage
  • allele combinations at several genes with inversion stay together for longer
  • combinations may iprove adaptation to certain niches
    • more likely to stay in that niche
    • coadapted gene complex
    • important in phenotypes that require several genes
      • behavioural traits
18
Q

inversions in Savannah and Mopti

A
  • tight linkage of different 2R chromosome inversion arrangements in these 2 groups with larval habitat preferences
  • suggests inversions contain coadapted gene complexes
19
Q

identifying polytene chromosomes

A
  • labour intensive in high numbers of mosquitoes
  • → PCR-based assays
    • amplify amplicon specific to one of two inversions (M or S)
    • high throughput probing of west african mosquitoes
  • some areas predominantly M, others S
  • different relative abundances in some areas
    • sympatric here
20
Q

M/S heterozygotes

A
  • virtually non-existent in wild populations despite overlap
  • heterozygotes are viable in lab
    • must be something upstream of mating acting as a reproductive barrier
    • pre-mating reproductive isolation
21
Q

speciation of M and S

A
  • at the early stage of speciation
  • will eventually form a separate species
  • almost fulfill biological species concept but can occasionally mate to produce viable offspring
  • S → A. gambiae ss
  • M → A. coluzzi
22
Q

kdr

A
  • knockdown resistance allele
    • pyrethroid resistance
  • originated and spread in S form
  • sympatric M forms took some itme to acquire it
  • then rapidly spread through M
  • gene flow is restricted
23
Q

mosquito genome

A
  • completed 2002
  • 250 Mb, 15000 genes
  • most sequences assembled onto chromsomes 2, 3 and X
  • 1000 mosquito genomes project
    • african mosquitoes sequenced to analyse gene flow and variation in isolated populations
  • 16 genomes project
    • evolution rates among different species
24
Q

genetics view

A
  • focus on linkage analysis using marker genes
  • small genetic markers ditributed among chromosomes
  • track which regions go to which individuals in offspring
    • which regions associated with certain phenotypes
25
Q

genetic markers

A
  • microsatellite repeats and SNPs
    • common, neutral, dispersed
    • high resolution mapping
  • >1 SNP per Kb ov Anopheles genome
  • microsatellite repeats
    • di/trinucleotides repeated multiple times
    • varies among individuals
  • identify with simple PCR
26
Q

linkage analysis

A
  • if an organism has obtained resistance:
    • some individuals have a variant conferring this that is found in a certain chromosome
    • = quantitative trait loci
  • different markers around that chromosome can vary between indiviudals
  • combination of markers = haplotype
  • cross individuals of different haplotypes
    • random combinations produced
  • markers consistently associated with resistance allele are physically much closer
    • use to identify allele location
    • more markers = higher resolution
    • SNPs also give higher resolution
27
Q

anopheles resistance phenotype mapping

A
  • isolation of 3 genomic regions with differential association with capacity to melanise parasites
28
Q

microsatellite distribution to identify resistance island

A
  • chromosome region 2L
  • wild caught mosquitoes with wild falciparum
  • microsatellites identified 15 Mb region
  • regions annotated as immune genes or as being upregulated during falciparum infection → candidate genes
  • 2 identified
    • APL1 and 2
29
Q

APL1/2

A
  • anopheles plasmodium-responsive leucine rich repeat 1 and 2
  • RNAi in infection experiments
    • only APL1 has an effect on oocyst number in rodent malaria
  • but may have an effect on natural transmission
30
Q

genomics view

A
  • involves e.g. microarrays
    • cheap, simple
  • spot gene probes with different coloured fluorophores onto plate
  • expose to RNA from infected and non-infected mosquito
  • hybridisation → fluorescence
  • differential expression identified by different colours
    • merge of 2 colours = both expressed
31
Q

RNAi

A
  • use in genome wide screening
  • target a gene to silence
  • observe change in phenotype
  • only a transient change
32
Q

CRISPR/cas9

A
  • easy, works in most organisms
  • cas9 nculease cuts to remove target
  • knockout by NHEJ or insertion by homologous recombination
  • may not always produce knockout
    • e.g. 3bp deletion has no frame shift, may not knockout

Vector-Parasite Interactions (6 decks)

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