Genetic Identification Of The Natural Citrus Hybrid-'Zigui' Tangor

Citrus is one of the most important fruit crops in the world. For citrus researchers, it is an important objective to breed elite citrus varieties desirable for the market. Recently, the breeders put more attentions on the agronomically important traits improvement including seedlessness and extending the maturity. Citrus germplasms, especially those with excellent quality, provide materials for citrus breeding and production development. 'Zigui' tangor is a natural citrus hybrid of unknown origin, morphology showed that this tangor possessed orange flesh and peel, seedlessness and early maturation. It showed high potential in citrus industry as a very early-mature variety. The purpose of this study is to identity the genetic background of 'Zigui' tangor and trace back to its parents, which could provide theoretic basis for exploiting and utilizing 'Zigui' tangor. Morphological markers, amplified fragment length polymorphism (AFLP), chloroplast simple sequence repeat (cpSSR) and nuclear simple sequence repeat (nSSR) were used for the genetic identification. The main results were as follows:(1) The phylliform index of 'Zigui' tangor (1.70) was significantly smaller than that of 'Hongju' tangerine (Citrus reticulata Blanco) (2.01) and 'Jincheng' orange [C. sinensis (L.) Osbeck] (2.15), but had no significant difference from that of 'Hamlin' sweet orange [C. sinensis (L.) Osbeck] (1.78). The stoma density of 'Zigui' tangor (314.25 Units/ mm~2) was the smallest compared with 'Hongju' tangerine (391.06 Units/ mm~2), 'Jincheng' orange (409.69 Units/ mm~2) and 'Hamlin' orange (470.21 Units/ mm~2) with significant difference. The stoma size (length and diameter) of 'Zigui' tangor was the smallest, but the ratio of length and diameter had no significant difference with that of 'Hongju' tangerine, 'Jincheng' orange and 'Hamlin' orange. Above results indicated that the leaves of 'Zigui' tangor were rounder than those of the tangerine and sweet orange, with the smallest stoma density and the stoma size, but the shape of stoma was almost the same as the other three varieties.(2) Pollen stainability of 'Zigui' tangor (44.04％) was between that of 'Hong' tangerine (71.1％) and 'Jincheng' orange (43.8％), and had significant difference with that of 'Hongju' tangerine, and also had insignificant difference with 'Jincheng' orange. Pollen diameter of 'Zigui' tangor (29.9μm) was the largest compared with 'Hongju' tangerine (25.7μm) and 'Jincheng' (26.4μm) orange, and the differences were significant. (3) According to scanning electron microscopy (SEM) analysis, we found that the pollen of 'Zigui' tangor was almost globose or fiat globose with netty sculpture, and similar with that of 'Jincheng' orange, 'Hongju' tangerine and 'Jinqianju' tangerine (Citrus reticulata Blanco). The pollen sculpture of 'Zigui' tangor resembled with that of 'Jincheng' orange, both with very wide ridge line and identical long horizontal inner apertures; However, the sculpture structure of 'Hongju' tangerine pollen was almost the same as that of 'Jinqianju' tangerine with extraordinary wide ridge line and round inner apertures of inequable sizes, so it was different from that of 'Zigui' tangor. Therefore, we could conclude that 'Zigui' tangor had close relationship with 'Jincheng' orange.(4) AFLP results showed that 'Zigui' tangor had similar bands with the sweet orange such as 'Jingcheng' orange and 'Taoyecheng' orange. Moreover, the AFLP clustering results showed that 'Zigui' tangor located between sweet orange and 'Hongju' tangerine. By cpSSR analysis, we found that cpDNA of 'Zigui' tangor was consistent with that of sweet orange and pummelo [C. grandis (L.) Osbeck], which inconsistent with that of mandarin and tangerine.Taken together, it could be concluded that the female parent of 'Zigui' tangor might be sweet orange. nSSR analysis showed that 'Zigui' tangor had the fewest bands in the TAA15 site and shared common bands with 'Taoyecheng' orange, 'Hamlin' sweet orange, 'Jingcheng' orange and 'Hongju' tangerine. Therefore, we deduce that the male parent of 'Zigui' tangor could be tangerine.In conclusion, the results verified that 'Zigui' tangor was a sexual hybrid between sweet orange (female parent) and tangerine (male parent).