Sex means any kind ofgenetic exchanges between two individuals. Also, it could be recognized as theoccurrence of meiosis. Most lineages across the eukaryote phylogeny havemaintained a form of asexual reproduction.
However, in more evolved lineagelike angiosperm in plants, vertebrate in animals, they perform sexuallyreproduction. During the sex process individuals could gain a set of benefitswhich include reducing inbreeding rate to maintain diversity, indirectlypreventing sudden environmental changes, and as a repair system. Mechanisms ofsex determination vary in different species. Basically, we divide them into twogroups: ESD (environmental sex determination) and GSD (genetic sexdetermination).
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 thewhole genome.An epigenetic process todetermine 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. Duringthe egg period, the changes in temperature would affect the gender of thosespecies. Temperature factors affect the expression of genes, enzymes, orhormones to determine sex. The proportion of males increases or decreases orpeaks at an intermediate temperature point (Warner & Shine, 2008).
Genetic sex determinationmeans 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 diploidfemales. For example, honey bee (Apismellifera), follows this mode. Within this type of sex determination, thehaploid males have only one single locus of CSD (complementary sex determiner),while females are heterozygous of this locus. The selection will prevent thediploid 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 belongsto an XY system (Sinclair, 1990).
The Sry gene controls downstream cascade toinfluence level of various sexual hormone, which will activate male-specificprogram and inhibit female development. Sry is an intronless gene encodingprotein with 204 amino acids. The product of this gene plays the role oftranscription factors to switch on the downstream reactions. The avian sexdetermination system is ZW (Smith, 2004), which means males are homozygous andfemales are homozygous. However, the molecular details for avian sex determinationremains to be discovered. Hitherto we still cannot make sure what gene exactlycontrols the sex.
Evolution of sex determination in SalicaceaeSalicaceae contains threegenera, Populus, Salix, and Chosenia. Allof species in this family are dioecious, which means they have both maleindividuals and female individuals. Many researches have revealed that thispattern is due to the common whole genome duplication event called ‘Salicoid’,which has generated the ancestor for all Salicaceaespecies (Tuskan et al. 2006; Dai et al. 2014). However, the sex determinationwith 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 isheterozygous (Yin et al., 2008).
The sex determination region is peri-telomericlocated on the of Chromosome XIX decided by sex-linked SNPs marker. Anotherpoplar 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 ofChr XIX. However, in Populus deltoides,the system changed into ZW, which means the sex determination locus presents onfemale 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 sexdetermination regions moved to the center of Chr XV. Since all species inSalicaceae were evolved from the same ancestor, the variety of sex determinationsystem within the family could be explained only by the presence of secondaryevolution after they diversified from each other.According to theassumption above, there is a question coming up naturally. What kind of dynamicdrove 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 maledetermination (which marked as XY system) from an ancestral Y chromosome to anautosome and then formed a neo-Y chromosome.
The force to promote this changeis sexually antagonistic selection, which has both theoretical and empiricalground.How did this mechanism drivethe transferring of sex determination locus? This event that forms a new sexchromosome is initiated by a mutation of sex determination locus on autosome.When followed by specific intensity of selection pressures on sex antagonisticgenes linked to both ancestral and new sex determination locus, a neo-sexdetermination locus could be fixed by the selection on sex antagonistic traits.
One way to examine thishypothetical model is to watch the recently derived sex-determining regions.The purpose is to see if those regions are associated with genes that aretargets of sexually antagonistic selection. This experiment was employed inpoecilid (Kallman et al., 1984) and cichlid (Lande et al., 2001) fishes.However, this only offered a weak support to this model.
A better way toexamine this hypothesis is to look for sexually antagonistic genes in veryyoung sex chromosomes, and in closely related species that locus transferringhas not happened already.Why we choose Salicaceae as target family?Salicaceaeoffers a perfect system to do research to check the assumption above. Firstly,species in this family are all dioecious with different sex determinationsystems 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 gendersdo have differences. This implies the presence of potential sex antagonistictraits which could firmly link to sex determination locus. Thirdly, the wholegenome sequencing data of Populustrichocarpa have been available since 2006 (Tuska et al., 2006), also thegenome sequencing and chromosome assembly was well performed for Salix suchowensis in 2014 (Dai et al.
,2014). Other genome sequencing data within Salicaceaeare continuously producing. The abundance of the genetic background researchoffers great reference for genomic study in this family. Last, both poplar andwillow are easy to reproduce by stem cutting cloning. Easily obtained materialsare convenient for further studies.GoalsThe species we used is Salix gooddingii which is a local arborspecies in North America belongs to Salixgenus. To test that model, we must solve following problems.
Firstly, are theredimorphism between male are female in Salixgoodingii? In other word, do gender affect any traits other than the sexualtraits? To answer this question, we should apply a well-designed experiment to measurea lot of characteristics including the height, diagram, dry weight, flowernumbers, wood properties, and stress resistance of a population. Those datawill be used for discovering whether there is dimorphism which is a signal forpotential sex antagonistic genes between male and female.Second problem is, arethose differences genetically tightly linked to sex determination region? Tosolve this problem, we first need to construct a genetic map for mapping genesto specific location. Also, we plan to employ sequence capture strategy toenrich fragments around sex determination region and detect SNPs in order tomap sex antagonistic genes near the SDR.
Last question is, if sexantagonistic genes are present, how to use the model to explain the evolutionof sex determination in Salicaceae? Besidethe sex antagonistic genes, we would like to combine the ‘Salicoid’ wholegenome duplication event as well as the rearrangement of chromosome after the WGDevent to revive the process of the evolution of sex determination inSalicaceae.