Advances of sex identification research of dioecious fruit trees

Abstract：In dioecious plants, both staminate and pistillate plants have unisexual flowers corresponding to genders. Although dioecious plant only shares a small proportion in angiospermae, it has always been an indispensable material for researchers to investigate plant evolution and separating mechanisms for sex expression due to its distinctive sex differentiation mechanism. In the life of perennials dioecious plants, the period before flowering, called juvenile phase, often costs a few years or even longer. Due to the long juvenile, it will cost several years to identify the gender phenotypes. Besides, the male and female plants always have different values in economy and ecology. Therefore, it is significant important to develop some methods for the detection of the two different flower types. Sex differentiation of plants is a special phenomenon of organogenesis, generally including female and male determination, gametophyte differentiation, and development and maturation process. Molecular level studies suggest that the sex differentiation of plants is a complicated process, related to the action of the induced signal transduction, the sex-determining gene have a derepression effect, making the special gene expresses selectively and then, the progress has actualized. Sex-determining genes play a decisive role in gender development. For plants containing sex chromosomes, they are the very position where their sex-determining genes are located on. The allosomes developed from autosomes. Recombinant inhibitors of chromosomal mutations (sex-determining genes) on sexual chromosomes in its early evolution are weak or even not yet occurred, and sex-determining genes are able to relocate by chromosomal recombination.Meanwhile, many studies have shown that plant hormones, genetic factors, epigenetic modification, etc.could determine sexuality of plants by the interaction between each other, and make gender phenotypes diversiform. After the attempt of early morphological gender identification studies of European poplars in the 1950 s, sex identification studies in other plants based on morphology have been carried out in succession. However, the results of these studies are easily influenced by the stage of plant age, nutriture, environmental conditions and many other factors. Thus, it could not be used as a reliable method to identify male and female phenotypes. Besides, studies based on the physiological and biochemical methods, isoenzyme analysis and other methods were carried out, but the results still do not have a high reference value. To be sure, all the differences in traits are derived from the differences in genetic material.Therefore, the most straightforward way to find differences in gender characteristics is to identify the differences in genetic levels between genders. In recent years, with the rapid development of molecular biology, more and more studies focused on early sex identification have been conducted using DNA based molecular markers. At the same time, this method has also provided us more rapid and accurate results. DNA markers are widely used in genetic diversity analysis, germplasm resources identification, genetic map construction, comparative genomics, phylogenetic studies and molecular marker assisted breeding and some other aspects. At present, many kinds of perennial dioecious plants have been analyzed using molecular marker to distinguish their sexuality in the early development stage using such a method. Lots of molecular markers have been successfully developed for the identification of plant gender by genomic sequencing and other technologies. Moreover, sex determination genes in some dioecious plants have been successfully mapped in corresponding chrosomes. Nevertheless, the problem about how to explore the gender-determining mechanisms through molecular markers and genderlinked regions is still troubling researchers in further research since most of these studies remained in the stage of searching and locating sex-linked genes using molecular markers. While molecular markers can only reveal part of the genetic mechanisms of gender differentiation, the cooperation of cell biology, biochemistry and other related technologies are very contributing. For DNA molecular marker technology in the study of plant sex identification, the operation is complicated, the screening of sex-specific bands is difficult, the universality of the plants in the same genus with different ploidy is poor, the reproducibility is not good, and the costs is high, all these problems have made its extensive application in the production practice hampered. There are also some problems in molecular sex-identification of plant species that has three gender types, including female, male and amphoteric plants. In future study, the function and interaction mechanism of various transcription factors in plant sex differentiation will be a hot issue, which means the importance of the cooperation of modern molecular biology techniques such as gene chip technology, RNA interference technology, the second or third generation sequencing technology and the other new technology. And these will promote the sex-identification progress in a highefficiency, high-throughput and high-accuracy way and can be finished by automation operations, which can meet the needs of production practice. We could predict with confidence that new DNA molecular marker technology and gene chip technology will unceasingly be advanced in future, and the genes related to sex-determination of dioecious plants will be found, cloned and obtained, providing new ways for early molecular gender identification.