The role of chromosome rearrangements
in reproductive isolation and speciation
(particularly in plants)
Coghlan et al. (2005)
Are chromosomal rearrangements merely a problem for the genome, or do they
have functional significance in the short term (e.g. by enabling a species to
adapt to changing environmental conditions) or in the long term (e.g. by
facilitating speciation)?
Problems
Both morphologically distinct species that lack chromosomal differences (e.g. translocations and
inversions) and morphologically cryptic species with chromosomal differences can be found
The amount of phenotypic evolution is not a good predictor of the
amount of karyotypic evolution
Are chromosome rearrangements important for
creating reproductive isolation barriers and speciation?
Or is the accumulation of chromosomal differences
between populations largely incidental to speciation?
M.J.D. White
YES!
Cryptic species with population-specific
chromosome rearrangements?
Grund et al. 2006, PNAS 103
Draba nivalis
See also:
Gustafsson ALS, Skrede I, Rowe HC, Gussarova G, Borgen L, et al. (2014) Genetics of Cryptic Speciation
within an Arctic Mustard, Draba nivalis. PLoS ONE 9(4): e93834. doi:10.1371/journal.pone.0093834
Grund et al. 2006, PNAS 103:
Although 99% of parental individuals were fully fertile, the
fertility of intraspecific crosses was surprisingly low. Hybrids
from crosses within populations were mostly fertile (63%), but
only 8% of the hybrids from crosses within and among
geographic regions (Alaska, Greenland, Svalbard, and Norway)
were fertile.
The frequent occurrence of intraspecific crossing barriers is not
accompanied by significant morphological or ecological
differentiation, indicating that numerous cryptic biological
species have arisen within each taxonomic species despite their
recent (Pleistocene) origin.
Draba nivalis
Cryptic species with population-specific
chromosome rearrangements?
Models of chromosomal speciation (Rieseberg 2001)
• Chain or Cascade models
• Chromosomal transilience model
• Monobrachial fusion model
• Recombinational model
• Quantum speciation model
• Stasipatric model
• Saltational model
References
White, M.J.D. (1978) Modes of Speciation.
Templeton, A.R. (1981) Mechanisms of speciation – a population
genetic approach. Annu. Rev. Ecol. Syst. 12, 23–48.
Baker, R.J. and Bickham J.W. (1986) Speciation by monobrachial
centric fusions. Proc. Natl. Acad. Sci. U. S. A. 83, 8245–8248.
Grant, V. (1981) Plant Speciation.
Lewis, H. (1966) Speciation in flowering plants. Science 152, 167–
172.
Fundamental feature of the models: chromosomal differences that have
accumulated between the neospecies and its progenitor(s) are assumed
to impair the fertility or viability of interspecific hybrids, thereby reducing
gene flow
Deviating features of the models:
• geographical isolation is (not) required for speciation
• the means by which chromosomal rearrangements arise and become fixed
• effects of rearrangements on the fitness of chromosomally heterozygous individuals
Chromosomal speciation: problems
• newly arisen chromosomal rearrangements
will exist in the population almost exclusively as
heterozygotes (inversion or translocation
heterozygotes)
• many chromosomal rearrangements have little effect on fertility
(ineffectiveness of chromosomal differences as barriers to gene flow)
• novel chromosomal arrangements have a selective disadvantage when they
first appear in a population:
the problem of underdominance: difficulties associated with fixing chromosomal
rearrangements that are strongly underdominant (i.e. reduce the fitness of
heterozygotes)
A paracentric inversion in D. pseudoobscura
• gross chromosomal rearrangements in Drosophila are well characterized as
rearrangements are easily detected in the chromosomes of their giant salivary glands
• the most common type of gross chromosomal rearrangement are paracentric inversions
(do not span the centromere)
• paracentric inversions are common polymorphisms in drosophilas and other fly species
(different populations of D. melanogaster harbor more than 500 inversion polymorphisms)
Chromosome speciation in Drosophila
Inversions are crossover suppresors –
evolutionary consequences (speciation)
Anopheles gambiae
(a) Polymorphic paracentric inversions in A.
gambiae chromosome arm 2R.
(b) Three non-interbreeding populations of A.
gambiae (named Bamako, Savanna and Mopti) that
live in the same region of Mali. The 3 populations
differ by chromosomal inversions that might be
contributing to speciation in A. gambiae. (For
example, a chromosome with arrangement 2R jcu has
inversions j, c and u on chrosmosome arm 2R.)
Do chromosomal rearrangements contribute to
speciation in Anopheles gambiae?
The role of chromosomal rearrangements in speciation in the A. gambiae species
complex is difficult to prove: even a highly significant coincidence in time between
chromosomal rearrangements and speciation does not prove a causal relationship.
• data consistent with expectations that inversions are involved in the
divergence process, elevated genomic divergence was found between Bamako
and Savanna in the regions carrying inversions
• but divergence was not observed throughout the inversions, rather it is
restricted to sites near the breakpoints
• surprisingly, the overall highest divergence was found on the X chromosome
• genes located on the sex chromosome may be the major force driving
speciation between the chromosomal forms of A. gambiae
Chromosome rearrangements provide large regions of the
genome protected from gene flow where isolating genes
may accumulate until complete reproductive barriers
exist.
Suppressed-recombination model
(Rieseberg 2001, Noor et al. 2001, Faria and Navarro 2010)
The model suggests that rearrangements may reduce gene
flow by suppressing recombination. CRs allow genes located
in these regions to differentiate, in contrast to genes in
freely recombining collinear regions.
Suppressed-recombination model (Rieseberg 2001, Noor et al. 2001)
Drosophila
- inversions have contributed to speciation between the close relatives D. pseudoobscura and
D. persimilis: inversions are found within the genomic regions associated with hybrid sterility
- chromosomal rearrangements reduce recombination between the genomes of the species,
thereby enabling genetic differences to accumulate within the rearranged regions
- inversions are more common between Drosophila species that are sympatric compared to
allopatric pairs
Plants
- the exact relationship between chromosomal rearrangement and speciation remains unclear
in plants! ...but more evidence is emerging (new methods available)
- seems that sunflowers (Helianthus) are only example: hybridization between two divergent
diploid species appears to have provoked speciation events in sunflowers (Loren Rieseberg’s
lab) ....and recently Mimulus guttatus (Lowry and Willis 2010)
perennial and annual plant
yellow monkeyflower (Mimulus guttatus)
M. guttatus: geographic distribution of the chromosomal inversion
(A) Map of western North America with the locations of populations of coastal perennials (blue), inland annuals (orange),
and inland perennials (purple), as well as obligate self-fertilizing species M. nasutus (yellow). (B) Marker order of the AN
and PE inversion arrangements along linkage group eight. Inland annuals and M. nasutus had the AN arrangement while
coastal and inland perennials all had the PE arrangement.
o a geographically widespread adaptive inversion polymorphism in the yellow
monkeyflower (Mimulus guttatus)
o the inversion is involved in a classic life-history shift in plants - an adaptive response
to differences in the seasonal availability of water resources:
- one arrangement of the inverted region is found in an annual ecotype that lives in
Mediterranean habitats characterized by reduced soil water availability in the summer;
- the other arrangement appears in a perennial ecotype that lives in habitats with high yearround
soil moisture.
o inversion polymorphism influences morphological and flowering time differences
between the two ecotypes = reproductive isolating barriers
o observation is consistent with the theory that adaptation to local environments can
drive the spread of chromosomal inversions and promote speciation.
o for the first time in nature was shown the contribution of an inversion to adaptation,
an annual/perennial life-history shift, and multiple reproductive isolating barriers
Inversion polymorphism and adaptation in Mimulus
When considering all the data, the emerging picture is that evidence
for chromosomal speciation varies between lineages. For example, it
is strong for Drosophila, but much weaker or absent in primates.
The observation that appears most consistently in many species is an
increased level of divergence near rearrangement breakpoints.
According to suppressed-recombination models, these regions
constitute strong candidate regions to accumulate alleles involved in
reproductive isolation.
Faria and Navarro (2010)
The question is, are CRs an important component of speciation?
• CRs can contribute to reproductive isolation through their effects on hybrid fertility, as
well as by reducing interspecific gene flow
• CRs most commonly contributing to reproductive isolation in plants are inversions and
translocations
• large translocations have the impact on the fertility of hybrids - 50% of gametes expected
to carry deletions or duplications of whole chromosome arms (or portions of arms).
Translocations also reduce recombination rates near centromeres or chromosomal
breakpoints due to mechanical interference with pairing and crossing over.
• The fertility effects of inversions are less severe, although for large inversions up to 50%
of gametes can be inviable due to recombination within the inversion resulting in
unbalanced gametes. However, inversions have little or no effect on fertility if
recombination is reduced (if the inversion is small, occurs in a low recombination region).
• Reduced recombination can contribute to the evolution of reproductive barriers by
facilitating the accumulation of hybrid incompatibilities or other key reproductive
barriers traits in the presence of gene flow.
Chromosomal rearrangements as post-pollination barriers