week 6- genome evolution Flashcards
what is the 1C-value
the amount of DNA in the unreplicated gametic nucleus
usually reported in terms of mass in picograms or the number of base pairs in gigabsepairs
1 pg = 0.978 Gb
what doe C in C value stand for
constant as this does not chnge between members of the species
explain C-value paradox
across eukaryotes 65000 x range in genome size
-some unicellular porozoans larger genomes than multicellular mammals
-plants with fewer cell types can have larger genomes than animals
-and organisms similar in complexity can have different genome sizes
what do people argue the c-value pradox arises from
varying amount of noncoding DNa such as repetitive DNA, introns in genes, regulartory elements of genes, pseudogenes etc.
now is suggested as a misundertanding of how evolution works
what do most eukaryotes have
small genomes, giant genomes are rare
what is variation in genome size driven by
the huge range in the amount of repetitive DNA, parasitic and “selfish” elements
what is repetitive DNA
DNA sequence motifs that are repeated hundreds or thousands of times across the genome, including tandem repeats (e.g., DNA satellites) and dispersed repeats (e.g., DNA transposons and retroelements)
what are 3 important evolutionary forces affecting genome size
-gene and genome duplications
-transposable elements
-deletions: some organisms are better at throwing out the trash
what are 4 types of DNA duplication
partial gene duplication, gene duplication, segemntal duplication, polysomy
what is homologous unequal crossing over
due to presence of highly similar sequences/repetitive elements
what is nonhomologous crossing over
short sequences that have some similarity mainly by chance
what do both types of crossing over result in
two chromosomal products, one that is longer (duplication) and one that is shorter than before (deletion)
what is paralogy
sequence homology due to dyplication
paralogous genes are homologous genes within the same species (genome)
what is orthology
sequence homology due to speciation
orthologous genes are homologous (corresponding) genes in different species
what might happen to a paralogous gene following gene duplication
may undergo neofunctionalisation
in which case both gene copies are maintained in duplicate in perpetuity because the loss of either copy would be detrimental to the organism
what is subfunctionalisation
expression divergence of the different copies
-different tissues
-different life stage
-different physiological conditions
what is euploidy
multiples of haploid set of chromosome
e.g. haploids, diploids, triploids, tetraploids, pentaploids, hexaploids
what are aneuploids
number of chromosomes is not exact multiple of haploid set
What is polysomy
duplication of at least one cmplete chromosome (a type of aneuploidy)
what is partial polysomy
duplication of a segment of a chromosome
what does chromosomal duplication or deletion lead to
does not contribute to animal evolution
often lethal or leads to infertility
in plants is polyploidy rare
It’s common, but permanent polysomy is hardly ever tolerated
what does polyploidisation consist of
addition of a whole genome set
gametes of a tetraploid are diploid
unevem numbers of chromosomes are a problem in meiosis: no reproduction
what is autopolyploidy
doubling of genome (single species of origin)
what is allopolyploidy
combination of the two genomes of two species
combination of unreduced gametes in hybridisation event
example of autopolyploidy
potato solanum tuberosum
-high levels of heterozygosity: max 4 alleles instead of 2
-higher fitness
-higher Ne (effective population size); hence selection stronger effect
-athough common: long evolutionary time not successful
-genetic imbalances, problems in meiosis
example of allopolyploidy
-results from interbreeding of different species
-much more common: 80% land plant
-present in various animal groups, but not common
-basically all land plants and in particular angiosperms have had several rounds of polyploidisation
-large proportion considered diploid
evidence for genome duplication
80 grass species, log scale, peaks in multimodal distribution indicated
allopolyploidy in agriculture
-domestiation over last 15000 years: most important crops often allopolyploidy
-Bread wheat, triticum aestivum, is hexaploid
