The main principle discussed in this paper would be the concept that sex
chromosomes are non-recombining; giving rise to the fact the Y-chromosome
undergoes degeneration over great time (fig.1). The phenomenon of degeneration is
induced by the accumulation of deleterious alleles (Muller’s ratchet), high mutation
rates, drift and selection. Degeneration is believed to be a characteristic of male
non-recombining sex chromosomes, in an attempt to preserve rare genes like the
sex determining SRY gene found on the human Y chromosome. Preventing
recombination preserves the gene, no recombination result in Muller’s ratchet and
since Muller’s ratchet is irreversible, disadvantageous alleles will be lost in the
absence of recombination. In this study however the sex chromosomes were
homomorphic, they were similar in morphology and therefore defied the theory that
non-recombining sex chromosomes degenerate over evolutionary time, it is also
worth mentioning that recombination of the male chromosomes ended before
species divergence. Recombination was frequently mentioned in the paper, as an
alternative reason for non-recombining sex chromosomes being homomorphic, since
gene shuffling is one of the main ways of limiting redundancy.
Figure 1 sex chromosome types
Adapted from Furmann, B., Sex Chromosome Evolution,
http://benjaminfurman.wordpress.com/research-interests/sex-chromosome-evolution/,
The paper explored the reasons for how non-recombining sex chromosomes of a
tree frog genus Hyla can be homomorphic when non-recombining sex chromosomes
are expected to degenerate over evolutionary time. Two hypotheses were proposed:
1. High-turnover hypothesis
Prior to the genes of the chromosome having time to degenerate new genes replace
them, thus adapting a high turnover rate and sustaining the morphology of both sex
chromosomes.
2. Fountain-of-youth hypothesis
The sex chromosomes remain “young” through recombination events between the
female X-Y chromosomes (reversed, hence male sex chromosomes: XX). Consistent
recombination generates more recent chromosomal arrangements.
Three frog species were analysed: Hyla arborea, Hyla intermedia and Hyla molleri.
These are three species of European tree frog. Sibship analysis of each species was
used to examine synteny between siblings of each sex. The frogs also had their cDNA
sequenced as a further comparison. A phylogeny was established using this
information, which was used to calculate the divergence point of H arborea from its
two sister species. Linkage maps were combined for the three species when
examining sex differences, as there was no difference in pattern between them.
Females were found to have higher rates of paired recombination than males in all
three species, with some areas of the male sex chromosomes completely absent of
recombination. This contrasted most female recombination rates, which were
between 0.3 and 0.5. This absence of male recombination is a tenet of the “fountainof-youth” hypothesis, and likely proves it. These distinct patterns were found at
several loci throughout the chromosome. Therefore, it was concluded that the three
species inherited their sex chromosomes from one ancestor. The divergence point of
H arborea was estimated to be during the late Miocene period, with different dates
for mitochondrial and nuclear divergence.