| Loquat(Eriobotrya japonica Lindl.)(2n=2x=34)is the Rosaceae loquat(Eriobotrya)plant,which is a special subtropical fruit trees in China,and the main cultivars are diploid.Its seeds are so large and numerous that causing a serious impact on the edible rate and the convenience of processing,therefore,seedlessness is the main goal of loquat breeding.The fertility of triploid loquat so low that triploid is the main way to achieve seedlessness.After years of selecting and cultivation,there are thousands of triploid loquat materials,among 3 loquat varieties has been certified.Triploid loquats always with lower fertility,but the Q24 line is special,it can get fruits under natural conditions with more seeds.Many studies have found that the most progenies of triploid plants are aneuploidy,aneuploidy also show more specific traits,for example dwarfing characteristics,which are high utilized value in production.Based on a large number of observations,we analyse systematically the fertility of the triploid loquat line Q24 and a preliminary study on the genomic characteristics,including the number of chromosomes and the distribution of molecular markers.The main results are as follows:1.Fertility of Q24(1)The fruit setting rate and the number of seeds have been counted under the natural condition of Q24 for many years.It is found that the fruit setting rate is similar to diploid loquat line Q14;Q24(the average number of seeds from 2016 to 2019 is1.09)has less seeds than Q14(average 4.84),but is similar to aother triploids in the number of seeds.However,selfing with bagging and emasculationq with bagging are all failed to obtain mature fruits.The diploid variety’Huabai 1~#’is used as the test parent,and the diploid line Q14 and the triploid line Q27 are used as control group to analyze the fertility of Q24.The results showed that Q24 and Q14(2x)is no significant difference in the fruit setting rate,but Q24 higher than Q27(3x);Q24 and Q14 showed extremely significant differences in the number of seeds,Q24 has fewer seeds in2017-2019.The pollen germination test showed that the pollen of Q24 was nonviable.So the male fertility of Q24 was low,and the female was fertile with the same fertility ability as Q14(2x).(2)Using the triploid line Q27 as a control,observe the meiotic chromosome behavior of Q24.The results showed that Q24 and Q27 had abnormal chromosomal behaviors in each periods.Q24(8.77±2.03)and Q27(9.70±1.70)are main trivalent chromosomes forms in metaphase and there is no significant difference between the two lines;in the first and second meiosis,both Q24 and Q27 have some chromosomes deviated from the nucleus,and there is no significant difference between the two lines.2.Chromosome number of hybrid progenies of Q24Counting the chromosome number of the progenies of Q24 as the female parent hybridize with Q14(2x),‘Ninghaibai’(2x),‘Huabai 1~#’(2x)respectively,and the natural progenies of Q24.The results show that there are more aneuploidies in the progenies of different hybrid combinations,and the distribution of the number of chromosomes is similar with 34-43;in total,there are 9 diploids and 1 triploid,93.5%of the plants are aneuploids;the number of chromosomes mostly is 37,accounting for16.1%,indicating that progenies of Q24 with higher tolerance to aneuploid.3.SSR marker development and Q24 specific marker screeningSSR molecular markers are abundant,highly polymorphic,reproducible and specific,and have wide applications in genetic map construction,assisted selection for breeding,and biogenetic diversity analysis.The genetic background of loquat is very narrow and there are less existing genomic SSR markers for loquat,the developed genomic SSR markers for loquat can be used for preliminary analysis of genomic characteristics of Q24.Analysis of the loquat(‘Seventh Star’)genome shows that there are a total of 184,045 SSR loci in the genome,and 178,126 primers pairs are designed,accounting for 96.78%.A total of 408 pairs of SSR primers are randomly selected,using the genomic DNA of‘Huabai 1~#’,‘Ninghaibai’,Q14 and Q24 as templates for PCR,of which 351 pairs amplified clear and stable bands,and 94 pairs of primers amplifies polymorphic bands among the four lines,accounting for 26.78%.In addition,102 pairs of reported SSR primers are screened.A total of 22 SSR primers are screened among 510 primer pairs that could specifically amplify Q24-specific bands.4.Hybrid identification and distribution of specific markers in the progenies of Q24(1)Q24 as the female parent are hybridized with Q14(2x),‘Ninghaibai’(2x),and‘Huabai 1~#’(2x).Using SSR markers to identify their true hybrids.The results show that the marker MK34283 can identify the hybrid progenies of Q24 and Q14 are all true hybrids;the marker NH033b can identify most of the hybrid progenies of Q24 and‘Ninghaibai’except for No.18,No.18 can be identified by the marker Hi07h02;the marker MK26044 can identify the most of the hybrid progenies of Q24 and‘Huabai 1~#’except for No.10 and No.34,No.10 and No.34 can be identified by the marker NH011b.(2)A large number of Q24 specific markers are missing in the hybrid progenies with‘Huabai 1~#’,‘Ninghaibai’and Q14.For example,the deletion rate of the molecular marker MK67181 in the progenies of Q24 hybridized with Q14 and‘Huabai 1~#’is more than 50%.Surprisingly,it is found that the specific bands of the male parent are also missing with high deletion rate.The 22 specific markers are annotated with gene functions,six are related to self-incompatibility genes,and two are annotated to the functions of zinc finger protein and lysine specific demethylase,and these three genes are correlated with plant fertility.In summary,the triploid line Q24 has very low male fertility,but female is fertile,and most of its progenies are aneuploids.There are abnormal behaviors in each periods of meiosis.The abnormal of meiosis will lead to change the structure of chromosome.Our research find that there exist some SSR markers missing in the hybrid progenies of Q24.We speculate that these missing markers may be related to the fertility and aneuploidy survival of Q24. |
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