| Casuarina equisetifolia is a perennial evergreen tree species,which belongs to Casuarina genus of Casuarinaceae.It is the firstly introduced species of Casuarinaceae plant with the largest artificial cultivation area in China.C.equisetifolia naturally occurrs in Australia,Southeast Asia and Pacific Ocean islands.Due to its outstanding physiological characteristics such as drought resistance,salt-tolerance,rapid growth,and barren tolerance,it can form pioneer communities with other tree species in difficult sites.It has become the important tree species of ecological shelter plantations,timber plantations and the multi-purpose plantations in coastal areas of southern China.In order to provide a theoretical basis for the construction and management of seed orchard and the artificial promotion of the natural regeneration of C.equisetifolia,the mating characteristics of the parents under different genetic distances,pollen competition,pollen dispersal pattern of C.equisetifolia and its effects on the adaptability and genetic diversity of offspring were studied in this paper using SSR molecular marker technology.The main research results are as follows:(1)The parental mating of C.equisetifolia under close or distant genetic distances would produce inbreeding depression and outbreeding depression.The effects of inbreeding depression of C.equisetifolia on the adaptability of the offspring are mainly manifested in cone development and seedling growth.The selfing progenies of C.equisetifolia would produce inbreeding depression.In addition to the seed germination rate,other offspring adaptability indicators,such as fruit-setting rate,seed-setting rate,the number of seeds,thousand-grain weigh,the plant height at five and fifteen months and survival rate in the field,etc.were significantly lower than those of inter-specific crossing,inter-provenance crossing and inter-family crossing treatments.There were no significant differences between inbreeding within the families and the selfing of C.equisetifolia in fruit-setting rate,seed-setting rate,indoor seed germination rate,plant height and DBH at fifteen months.But the the number of seeds,thousand-grain weigh,the seedling height at five months and survival rate in the field of progenies produced through inbreeding within the families of C.equisetifolia were significantly higher than those of selfing progenies.There is a certain degree of outbreeding depression in inter-specific crossing of C.equisetifolia,and its influence on the adaptability of offspring is mainly manifested in seed germination and early seedling growth.The seed germination rate of inter-specific crossing progenies between C.equisetifolia and C.glauca was very low,only 3.0% and 2.7%,and obviously lower than the other four crossing treatments.And the seedling height at 5 months was significantly lower than that of inter-provenance crossing and inter-family crossing,but higher than those inbreeding within the families and that selfing of C.equisetifolia.However,there were no significant difference among the progenies of inter-specific crossing,inter-provenance crossing and inter-family crossing in seedling height and DBH at 15 months in the field.And all of them were higher than the the progenies of inbreeding within the families and selfing of C.equisetifolia.(2)To a certain extent,pollen competition can alleviate the impact of inbreeding depression on seedling growth,but can not completely eliminate the depression of progeny adaptability.During pollen competition test,when pollen of the same species or a close genetic distance to female parent was mixed,C.equisetifolia would produce more seeds.The number of seeds produced by each treatment(pollens from different male plants were mixed in equal amounts)in descending order was treatment 5(Provenances+Families)> treatment 4(C.equisetifolia + C.glauca+ C.cunninghamiana)> treatment 6(Provenances+self)> treatment 1(C.equisetifolia+C.glauca)> treatment 3(C.glauca+ C.cunninghamiana)> treatment 2(C.equisetifolia + C.cunninghamiana).However,the performance of the seed germination rate of each pollen competition treatment was not consistent with the performance of the number of seeds.The seed germination of intraspecific pollen competition test is significantly lower than interspecific pollen competition test.The order in seed germination rate of each treatment was treatment 3>treatment 4>treatment 1>treatment 2>treatment 6>treatment 5.The seedling height at 5 months of treatment 3 was the largest,which was 56.8cm,significantly higher than that of all other treatments,showing obvious heterosis.Treatment 6 showed significant inbreeding depression in germination rate,survival rate and plant height.Treatment 5 was significantly higher than treatment 6 in survival rate and plant height,and had no significant differences with treatment 1and treatment 2.In the inter-specific pollen competition experiment of C.equisetifolia with different genetic relationships,the pollens,which had closer genetic relationships to the female parent,had obvious competitive advantages in the mixed pollen competition.Most of the pollen sources of the progenies were C.equisetifolia and C.glauca.However,in the intra-specific pollen competition test,the pollen from close genetic distance to female may be eliminated and the pollen sources of progenies tended to be the male plants of different provenances in order to avoid inbreeding depression.(3)The open-pollination progeny population of C.equisetifolia had high genetic diversity.And the effective distance of pollen dispersion was 70 m,and the pollen dispersal curve conformed to the power function thick with tail model.Totally 123 alleles were detected from the open-pollination progeny population using 12 SSR markers.The average number of allele and effective number of alleles were 10.25 and 3.338.The average observed heterozygosity,the average expected heterozygosity,the polymorphism information content,the average Shannon’s index and Nei’ s genetic diversity index were 0.667,0.677,0.629,1.366 and 0.677,respectively.Under the 95% confidence interval,the paternity could be determined for 162 progenies,and the identification rate was 36.16%.Under the 80% confidence interval,the pollen source of 311 progeny were identified,accounting for 69.42% of the total progeny population.The paternity analysis of open-pollination progeny of C.equisetifolia showed that the effective pollen dispersal distance of C.equisetifolia was 0-70 m,and the mean pollen dispersal distance was 28.74 m.The pollen from male plants near the female had obvious advantages in pollen contribution rate of progeny.96.42% of progenies with confirmed paternal parents settled in the range of 60 m around the mother tree.With the increase of the parental distance,the pollen contribution rate of the male parent decreased significantly.There were significant correlation among the progeny adaptation indexes,but no correlationship between the progeny growth traits and genetic diversity of C.equisetifolia was found.(5)It was found that natural regeneration population of C.equisetifolia can maintain a high genetic diversity.And the seed dispersal of C.equisetifolia was short-distance dispersal.The average expected heterozygosity,observed heterozygosity,polymorphic information amount and Nei’s genetic diversity index of natural regeneration population were 0.72,0.73,0.69 and 1.72,respectively.At 80% confidence interval,female parents of 148 offspring which accounted for72.20% of the total offspring number were identified using genotyping data obtained from 11 SSR markers.The seed effective dispersal distance of C.equisetifolia ranged from 10 m to 130 m,with an average of 71 m,which was categorized as short-distance dispersal.The identified female parents were mainly positioned at the northeast and east of the sample plot,and their offspring supply reached up to 75.00%.It is expectable to achieve the goal of artificially promoting the natural regeneration of C.equisetifolia seedling plantation using some auxiliary regeneration measures. |
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