How do reproductive barriers evolve?
The development of reproductive isolation (speciation) is a key
event in evolution, and hence for the origin and maintenance of the
tremendous diversity of life on earth. For sexually reproducing
organism there is general agreement that reproductive isolation is
a necessity for divergence. Nevertheless, the mechanisms involved
in the formation of reproductive barriers are poorly understood.
For organisms where reproduction and recombination do not
necessarily coincide, species concepts) remain unresolved.
For prokaryotes, in particular, taxonomic boundaries are obscure
and it is questionable if a species concept in terms of
reproductively isolated units is meaningful (cf. Colloquium 3). In
order to understand the high levels of homologous recombination
among Thermotoga strains across large geographic and phenotypic
distances, global population genetic approaches are needed. A large
number of Thermotogales isolates will be sampled worldwide and used
in multilocus-sequence-typing and-analysis (MLST; closely related
strains, and MLSA; more distantly related isolates). We will
investigate population structure and how species-like boundaries
(if present) may form in the face of high levels of recombination.
In the cases of Yersinia and Salmonella we will explore the
evolution of distinct taxonomic units within each genus, focusing
on pathogenic strains. This work will be carried out using
available DNA sequence data and a bioinformatic approach.
Some passerine birds coexist in certain regions as species that
are distinct, but not completely reproductively isolated (hybrid
zones). Thus the evolution of species boundaries is still in
action. When hybrids are unfit, females should avoid mating with
males from the other species. Different signals may provide
information of varying reliability regarding species identity and
mate quality. We will expand on optimality models developed for a
one-species setting to investigate how information content of
sexual signals and relative abundance of con- and heterospecifics
may affect mate search strategies of females in a hybrid zone and
test specific predictions from the models using experimental
studies. We will further explore the often-neglected alternative
hypothesis that intersexual competition may account for
unidirectional hybridization.
For pied and collared flycatcher, it has been documented that
unfit hybrids can reinforce prezygotic isolation, for instance
through sympatric divergence in traits used in mate recognition.
Prezygotic isolation includes lack of sexual attraction between
members of differentiated populations and often involves
sex-specific traits controlled directly or indirectly by genes
linked to the sex chromosomes. We suggest that sex chromosome
evolution and the particular architecture of sexlinked genes
(hemizygosity and reduced rate of recombination) play a crucial
role in speciation. We will investigate speciation traits using
genome-wide genotyping of various hybrid and backcrossed birds, and
by correlating genotypes with phenotypic traits, such as fertility
and viability (post-zygotic isolation traits), mate preferences and
sexual signals (pre-zygotic isolation traits).
In the European grayling system several viable demes have been
established within 25 generations. The population structure may be
a result of founder events (non-random colonization), selection for
spawning time (variable environments leading to isolation-bytime)
or selection against dispersal (isolation-inspace). We aim to
identify the ecological factors that facilitate the strong
selection. We will then perform in-situ testing for pre-zygotic
isolation mechanisms (mate choice experiments, timing of
maturation) and post-zygotic isolation mechanisms (early
development, selection against hybrids). Reproductive isolation may
follow from development of phenotypic differences. This will be
studied by estimating field-based variance-covariance matrices for
important traits.
Through polyploidization, reproductive isolation (i.e., new
species) may arise in a single or few generations. In certain
plants and invertebrates the frequency of polyploids is
particularly high in harsh environments such as the Arctic, where
asexuality is also common. Polyploidy is thought to be important
for maintenance of genetic diversity in the absence of
recombination and could also enhance expression of genes. By
comparative studies of a range of species, including both diploids
and polyploids, we will assess whether particular ecological
settings favour the establishment of polyploids and test whether
polyploids are more diverse and fitter than diploids in the Arctic.
Populations from a wide latitudinal range will be screened for
ploidy-level, heterozygosity and for correlations between
ploidy-level and important life history (e.g., growth rate, size)
and related features (e.g., RNA/DNA-ratios). For Arctic plants with
both diploid and polyploid cytotypes genetic and phenotypic
charac
