Sex means any kind of
genetic exchanges between two individuals. Also, it could be recognized as the
occurrence of meiosis. Most lineages across the eukaryote phylogeny have
maintained a form of asexual reproduction. However, in more evolved lineage
like angiosperm in plants, vertebrate in animals, they perform sexually
reproduction. During the sex process individuals could gain a set of benefits
which include reducing inbreeding rate to maintain diversity, indirectly
preventing sudden environmental changes, and as a repair system. Mechanisms of
sex determination vary in different species. Basically, we divide them into two
groups: ESD (environmental sex determination) and GSD (genetic sex
determination). Within GSD, the sex determination still has a number of forms.
It could be a tiny difference of one single nucleotide, or the ploidy of the
whole genome.

An epigenetic process to
determine the sex can be triggered by environmental changes. For reptiles like lizards
(Charnier, 1965), turtles (Pieau, 1972), and crocodilians (Lang & Andrews,
1994), they are the first discovered environmental sex determination system. During
the egg period, the changes in temperature would affect the gender of those
species. Temperature factors affect the expression of genes, enzymes, or
hormones to determine sex. The proportion of males increases or decreases or
peaks at an intermediate temperature point (Warner & Shine, 2008).

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Genetic sex determination
means male and female individuals have genetic differences. In hymenopterans,
including ants, wasps, and bees, sex is determined by ploidy level.
Unfertilized eggs form haploid males while fertilized eggs form diploid
females. For example, honey bee (Apis
mellifera), follows this mode. Within this type of sex determination, the
haploid males have only one single locus of CSD (complementary sex determiner),
while females are heterozygous of this locus. The selection will prevent the
diploid males since diploid males are sterile. In mammals, like mice and human,
our sex is determined by the Sry gene located on the Y chromosome which belongs
to an XY system (Sinclair, 1990). The Sry gene controls downstream cascade to
influence level of various sexual hormone, which will activate male-specific
program and inhibit female development. Sry is an intronless gene encoding
protein with 204 amino acids. The product of this gene plays the role of
transcription factors to switch on the downstream reactions. The avian sex
determination system is ZW (Smith, 2004), which means males are homozygous and
females are homozygous. However, the molecular details for avian sex determination
remains to be discovered. Hitherto we still cannot make sure what gene exactly
controls the sex.

Evolution of sex determination in Salicaceae

Salicaceae contains three
genera, Populus, Salix, and Chosenia. All
of species in this family are dioecious, which means they have both male
individuals and female individuals. Many researches have revealed that this
pattern is due to the common whole genome duplication event called ‘Salicoid’,
which has generated the ancestor for all Salicaceae
species (Tuskan et al. 2006; Dai et al. 2014). However, the sex determination
with family varies a lot. For one poplar species, Populus trichocarpa, the first sequenced tree species in the world,
its sex determination type is XY system which means the male one is
heterozygous (Yin et al., 2008). The sex determination region is peri-telomeric
located on the of Chromosome XIX decided by sex-linked SNPs marker. Another
poplar species, Populus balsamifera,
follows the same pattern of Populus trichocarpa,
with sex determination region located near telomere of Chr XIX (Geraldes et al.,
2015). The position of sex determination region has changed in Populus tremuloides (Pakull et al., 2009,
2011, 2015; Kersten et al., 2012, 2014), which is located near the center of
Chr XIX. However, in Populus deltoides,
the system changed into ZW, which means the sex determination locus presents on
female individuals (Yin et al., 2008). For the other genus Salix, species Salix suchowensis (Hou et al., 2015;
Chen et al., 2016) and Salix viminalis
(Pucholdt et al., 2015), based on currently marker linked analysis, their sex
determination regions moved to the center of Chr XV. Since all species in
Salicaceae were evolved from the same ancestor, the variety of sex determination
system within the family could be explained only by the presence of secondary
evolution after they diversified from each other.

According to the
assumption above, there is a question coming up naturally. What kind of dynamic
drove the evolution of those sex determination systems in Salicaceae? Based on a module study (Van Doorn & Kirkpatrick,
2007), we suggested a mechanism that can explain the movement of male
determination (which marked as XY system) from an ancestral Y chromosome to an
autosome and then formed a neo-Y chromosome. The force to promote this change
is sexually antagonistic selection, which has both theoretical and empirical
ground.

How did this mechanism drive
the transferring of sex determination locus? This event that forms a new sex
chromosome is initiated by a mutation of sex determination locus on autosome.
When followed by specific intensity of selection pressures on sex antagonistic
genes linked to both ancestral and new sex determination locus, a neo-sex
determination locus could be fixed by the selection on sex antagonistic traits.

One way to examine this
hypothetical model is to watch the recently derived sex-determining regions.
The purpose is to see if those regions are associated with genes that are
targets of sexually antagonistic selection. This experiment was employed in
poecilid (Kallman et al., 1984) and cichlid (Lande et al., 2001) fishes.
However, this only offered a weak support to this model. A better way to
examine this hypothesis is to look for sexually antagonistic genes in very
young sex chromosomes, and in closely related species that locus transferring
has not happened already.

Why we choose Salicaceae as target family?

Salicaceae
offers a perfect system to do research to check the assumption above. Firstly,
species in this family are all dioecious with different sex determination
systems which could contain the ancestral type and the ‘new’ type. Secondly,
even if the dimorphism in two genders is not obvious to see, those two genders
do have differences. This implies the presence of potential sex antagonistic
traits which could firmly link to sex determination locus. Thirdly, the whole
genome sequencing data of Populus
trichocarpa have been available since 2006 (Tuska et al., 2006), also the
genome sequencing and chromosome assembly was well performed for Salix suchowensis in 2014 (Dai et al.,
2014). Other genome sequencing data within Salicaceae
are continuously producing. The abundance of the genetic background research
offers great reference for genomic study in this family. Last, both poplar and
willow are easy to reproduce by stem cutting cloning. Easily obtained materials
are convenient for further studies.

Goals

The species we used is Salix gooddingii which is a local arbor
species in North America belongs to Salix
genus. To test that model, we must solve following problems.

Firstly, are there
dimorphism between male are female in Salix
goodingii? In other word, do gender affect any traits other than the sexual
traits? To answer this question, we should apply a well-designed experiment to measure
a lot of characteristics including the height, diagram, dry weight, flower
numbers, wood properties, and stress resistance of a population. Those data
will be used for discovering whether there is dimorphism which is a signal for
potential sex antagonistic genes between male and female.

Second problem is, are
those differences genetically tightly linked to sex determination region? To
solve this problem, we first need to construct a genetic map for mapping genes
to specific location. Also, we plan to employ sequence capture strategy to
enrich fragments around sex determination region and detect SNPs in order to
map sex antagonistic genes near the SDR.

 

Last question is, if sex
antagonistic genes are present, how to use the model to explain the evolution
of sex determination in Salicaceae? Beside
the sex antagonistic genes, we would like to combine the ‘Salicoid’ whole
genome duplication event as well as the rearrangement of chromosome after the WGD
event to revive the process of the evolution of sex determination in
Salicaceae.

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