Ever-Young Sex Chromosomes in European Tree Frogs
The decaying Y chromosome of mammals began when a new masculinizing gene (SRY) first appeared on an
autosome. Recombination stopped in males in areas around the SRY gene region. Several theories have been
suggested to explain the cause of the degeneration of Y chromosomes. Stock et al. (2001) explain that genes
trapped in the non-recombining segment accumulated deleterious mutations under the combined forces of
genetic drift, selective sweeps, background selection, and Muller’s ratchet. The study also highlighted
recombination in XY females should oppose Muller’s ratchet and prevent the evolutionary decay of sex
chromosomes. Local gene conversions found between X and Y chromosomes in mammals supports,
occasional X-Y recombination occurring either in males or sex-reversed XY females. On the other hand given
the findings of high rate of female recombination, a wide variety of new Y haplotypes would be generated by
a single event of sex-reversal. Whilst there is no male recombination, those haplotypes eliminated of the
deleterious mutations but still have male-advantageous alleles, should be sorted by natural or sexual
selection and spread among natural populations within a few generations. The study found a significant sex
differences in allelic frequencies, which could be explained by this interaction between recombination and
selective sweeps, despite low sequence differentiation found between X and Y chromosomes.
Sex chromosome homomorphy may results from a high rate of turn over events where master sexdetermining genes may become replaced by new ones on other genes. According to this “high-turnover”
hypothesis, sex chromosomes are replaced before they had time to decay. Alternatively the X-Y similarity
could be due to occasional recombination events, occurring in sex-reversed XY females. This is known as the
“fountain-of-youth” hypothesis and suggests it is by these occasional recombination events that sexchromosome integrity can be maintained over long evolutionary times. Stock et al. (2011) tests between
these two hypotheses by investigating gene genealogies and recombination patterns from three sister
species of European tree frogs.
The European tree frogs used in this study were H. arborea and two of its sister species H. intermedia and H.
molleri. The resource pedigree consisted of 2,863 individuals from 111 known family groups, each including a
mother, father, and an average of 20 offspring per family. Once tadpoles had grown (one per mating pair) in
the laboratory, the tips of the tail were used for tissue sampling and to prepare DNA samples. Phylogenetic
analyses of mitochondrial and nuclear genes showed that H. arborea diverged from the two sister species.
Recombination maps for sex-linked markers revealed the complete absence of recombination in males
contrasting to high recombination rates found in females. Higher X-Y similarity was found within the three
species than between them.
The study found all three species inherited the same pair of XY sex chromosomes from a common ancestor.
Regardless of no recombination in males, Y chromosomes show higher sequence similarities. These findings
imply sex-chromosome homomorphy in H. arborea is not due to a recent turnover event. Local gene
conversion between X and Y chromosomes were found to occasionally occur in mammals. Though this does
not necessarily account for all species, it does support the hypothesis of occasional X-Y recombination
occurring in males or in sex-reversed XY females. Neither of the hypotheses could exclusively be accepted.
This is due to the potential for turnover events to occur from the same mechanism causing sex reversal and
X-Y recombination. The turnover events could be via sex-ratio selection, sex-antagonistic selection, etc.
Differential gene exchange between parapatric morphs of Littorina saxatilis detected using AFLP markers
This article has explored many topics that we have touched upon in this module and previous modules on
evolution. This paper is mainly concerned about speciation and selection. It poses the question as to
whether speciation is occurring due to acquired incompatibility due to a physical barrier, or whether it’s due
to disruptive selection which commences with the differentiation of a specific and restricted set of gene loci.
Selection is known to play a pivotal role during speciation by ultimately favouring advantageous
characteristics in species, and consequently eliminating those that seem to hinder the fitness of individuals
of a species. As a result we see an increase or decrease of particular traits in populations and species overall.
The paper also looks at genetic divergence as well as genetic linkage. The paper discusses simple models of
speciation, where it’s explained that genetic divergence could have been initiated by disruptive selection, in
conjunction with strong selection and either pleiotropy (one gene affecting multiple phenotypic traits), or
the linkage of genes (where genes are located closely on a chromosome and are said to be transmitted
together during meiosis).
In their study, they used a fixation index (FST) which measures population differentiation due to difference in
genetic structure, using polymorphism data. Their study compared the FST values for 306 amplified fragment
length polymorphism (AFLP) and found that around 5% of the loci showed a greater differentiation than
expected, meaning that selection has been more effective than originally thought.
The paper argues that the pattern detected by the tests may be due to something that wasn’t considered
before such as introgression; genetic information transferring from one species to another due to
hybridization and repeated backcrossing. Therefore the alternate hypothesis would be that speciation
occurrence has no link to the separation of a population physically or through the action of disruptive
selection.
Data was collected from the rocky shores from 5 sites. Snails were collected from two locations on each site,
one where the H morph was found and one where the M morph was found. They were characterised by
their visual differences. The genomic data of the snails was taken from the head-foot tissue and purified.
AFLP analysis was used to produce the results and to analyse band presence or absence. There were high
levels of polymorphism at 94.8%. There were at least 5% of the loci studied that seemed to be under
selection or linked to loci that are. This implied that there is a general barrier to gene exchange between H
and M populations that is greater than that would be expected from their spatial separation.
AFLP data only shows the polymorphisms as presence or absence polymorphisms. The frequency of the band
presence was estimated from the presence/absence matrix for each sample. Hardy-Weinburg equation was
used for genotypic frequencies, with band presence being dominant over absence. The allele frequency was
determined from the AFLP data to look at the loci differences and determine the level of polymorphism and
whether it relates to an actual barrier to gene exchange. FST values were produced from a simulation model
to compare the observed results to. The observed results were compared to the 0.99 quantile of the
simulated values, and if the loci was outside of this region, the loci would be linked to selection.