Research advances on the genetic relationships of kiwifruit (Actinidia Lindl.)

Abstract: Kiwifruit is an important fruit tree resource, and is one of the most successful fruit tree species for large-scale commercial cultivation worldwide. The natural distribution of kiwifruit is very extensive, but most of its taxa are mainly concentrated in the area south of the Qinling Mountains and east of the Hengduan Mountains. Chinese kiwifruit is not only a variety, but also different ecological habits.Chinese kiwifruit is rich in germplasm resources, contains a large number of wild resources, and has high genetic diversity, which can provide rich genetic basis and material conditions for the breeding of high-quality cultivars. Germplasm resources research has always been the most important foundation for fruit tree research, especially for breeding. The research on genetic relationship has become the core content of the research and utilization of fruit tree germplasm resources, which can provide a scientific basis for further exploration of species origin and evolution, systematic classification, germplasm conservation and utilization, and fruit tree breeding. Domestic and foreign scholars have studied the genetic relationship of kiwifruit germplasm resources from morphology, palynology, cytology, biochemical analysis to DNA molecular markers, from macroscopic level to microscopic level. The micromorphological characteristics, body size, cell structure and density of the leaf epidermal hair of the kiwifruit werehigh polymorphism, which can be used as evidence for species identification. At present, the classification system of kiwifruit plants is mainly based on the type and degree of the indumentum of the fruit and leaves, the spots on the surface of the fruit and the morphology of the medulla of branches, and also giving consideration to the geographical distribution and other factors, as well as the combination of quantitative classification and computer processing methods. Liang C. F. divided the Chinese kiwifruit germplasms into four sections: Sect. Maculatae, Sect. Leiocarpae, Sect. Stellatae and Sect. Strigosae.The Sect. Leiocarpae can be divided into the Ser. Lamellatae and the Ser. Solidae, and the Sect. Stellatae can be divided into the Ser. Imperfectae and the Ser. Perfectae, which constitutes the current taxonomic system of four groups and four systems of Actinidia. However, the morphological characteristics of plants are susceptible to environmental conditions, and there are many subjective factors, which may lead to the divergence of taxonomic opinions. The morphological characteristics of kiwi pollen are controlled by genotypes, which are highly genetic conservation and are not affected by external conditions and carry a large amount of information. Related researchers used scanning electron microscopy to observe the pollen morphology of different species of kiwifruit. They found that the outer wall of the kiwi pollen has a large difference and has a significant polymorphism. They believe that the pollen grains of the genus have 3-colporate or 3-like colporate, which can be used as an important basis for packet classification. Kiwifruit has a wide range of chromosomal ploidy variations. Under natural conditions, diploid, tetraploid, hexaploid, octaploid and even dodecaploid occur, and the distribution pattern is reticulate the intraspecific and interspecific, as well as the sympatric distribution of multi species to various degrees. These make interspecific hybridization and gene introgression frequent, resulting in a series of rich genetic variations. Therefore, in-depth study on the interspecific and intraspecific genetic relationship of kiwifruit plants provides an important basis for breeding new varieties. The karyotype analysis of different ploidy revealed that the chromosome base of kiwifruit was 29, but there is still no clear conclusion about its chromosome origin. The research on the isozyme level shows that the kiwifruit has high genetic diversity at the level of cultivars and species, and there is a high degree of genetic heterozygosity and multiple alleles heterozygosity at its isozyme sites. At the DNA level, researchers used various methods of DNA molecular markers to detect different kiwi germplasm resources. The results showed that kiwifruit species or cultivars had rich genetic diversity, and there were high genetic diversity among different geographical regions. The genetic distance between kiwifruit cultivars was related to geographical distribution. In addition, studies on microsatellite showed that most kiwifruits showed higher heterozygosity, and polyploid kiwifruit had more genetic diversity. In addition to studying nuclear DNA, chloroplast DNA(cpDNA) and mitochondrial DNA(mtDNA) have also received attention in kiwifruit research. The results show that the cpDNA of kiwifruit plants has strict paternal inheritance,and mtDNA has strict maternal inheritance. This rare genetic model complicates the genetic relationship between kiwifruit plants to a certain extent. In general, the research on the genetic relationship of kiwifruit has made great progress, especially the continuous improvement and mutual compensation of various molecular markers in recent years, making the research and analysis of genetic relationship more accurate and reliable. However, there are still many problems to be solved in the study of the genetic relationship of kiwifruit germplasm resources. In the future research, with the continuous innovation of research methods and the deepening of research content, the research on kiwifruit germplasm resources,especially the genetic relationship research, will surely achieve more results. It is of great scientific significance and application value for the rational utilization of abundant kiwifruit germplasm resources in China, especially the exploitation and utilization of a large number of excellent wild or semi-wild resources, and the breeding and industrial development of new kiwifruit varieties.