Triploid Production Research Articles

Induction and viability of tetraploids in brook trout (Salvelinus fontinalis)

Brook trout (Salvelinus fontinalis) populations are threatened by introduction of invasive species, habitat loss, and habitat degradation in their native range and are a problem invasive species in western Unites States and Canada and in Europe. Stocking sterile triploids has been promoted as an approach to reduce negative effects of stocking of brook trout for recreational fishing on native fish populations. Crossing a tetraploid with a diploid is a method of triploid production that may help hatcheries meet demand. We induced tetraploidy in brook trout by application of 633 kg·cm−2 of hydrostatic pressure for 8 min at 70%–72.5% of the first cleavage interval. Yields of above 50% tetraploid progeny at hatching were readily achieved, although few animals reached 1 year of age. We crossed a male tetraploid with female diploid fish and produced interploid-triploids with eyeing rates in excess of 50%, demonstrating male tetraploids are fertile and capable of siring triploid progeny. Female tetraploid fish were reared to 16 months posthatching and possessed follicles in secondary vitellogenesis, suggesting tetraploid females are also fertile. Tetraploid induction rates in excess of 96% were achieved applying the same hydrostatic pressure treatment to zygotes of tetraploid × diploid crosses at 30 min postfertilization.

Gamete quality in triploid Pacific oyster (Crassostrea gigas)

Triploidy induction in shellfish aims to obtain faster growth and sterility of reared individuals. Triploid Pacific oysters are most often not totally sterile, but have greatly reduced reproductive capacities compared to those recorded in diploid individuals. The description of gamete output in triploid aquatic animals is most often confined to the assessment of fecundity in females. The present work aims at further describing gamete quality of triploid Pacific oysters and comparing it to values observed in diploid individuals. Diploid and triploid oysters (produced by mating tetraploid males×diploid females) were reared in standard conditions and then transferred to the wild to allow the offspring to attain reproductive maturity. At the end of gametogenesis, the reproductive characteristics of both groups were estimated.After gonad sampling, gametes could be observed in 92.9% diploid oysters compared with 42.0% in triploids. A higher number of spermatozoa was produced in diploids (344±106×109 spermatozoa) compared with triploids (5±7×109 spermatozoa). Furthermore, the percentage of motile sperm and sperm swimming speed were higher in diploids compared with triploids. In contrast, a higher intracellular ATP content was found in triploids (99.1±34.0nmole10−9 sperm) than in diploids (63.6±20.7nmole10−9 sperm). A higher number of oocytes was collected from diploid oysters (19.1±3.8×106 oocytes), than from triploid ones (0.1±0.1×106 oocytes). The D-larval yield was 45% higher for crosses (female×male) triploid×diploid, than the control (diploid×diploid). Furthermore, the lowest D-larval yield was measured for triploid×triploid crosses.Considering the present data, a partial estimation of the reproductive potential of triploid Pacific oysters (triploid×triploid crosses) could be close to 0.06% of that of diploid individuals. However, this estimation is probably over-evaluated because it does not take into account the low sperm production of triploids (1.5% compared to diploids), the unknown frequency of spontaneous spawning in triploids and the low viability of the progeny. The consequences of the low reproductive potential of triploid Pacific oysters on natural populations and on hatchery practices are discussed. In conclusion, the present work confirms that triploidy leads to a limited reproductive potential as estimated by gamete characteristics and embryo developmental success. Statement of relevanceThis study is relevant to aquaculture because triploids are produced by aquaculture and the consequences of their low gamete quality, described in this paper, on the natural environment but also on hatchery practices are discussed.

HIGHLIGHTS OF THE UNIVERSITY OF FLORIDA, CITRUS RESEARCH AND EDUCATION CENTER'S COMPREHENSIVE CITRUS BREEDING AND GENETICS PROGRAM

A comprehensive citrus breeding and genetics program was established at the UF/CREC during the mid-1980s combining conventional breeding with tools provided by tissue culture, emerging biotechnologies, molecular genetics and genomics. Scion Improvement: The Florida citrus industry is 90% processing sweet oranges. A major goal has been to develop new cultivars for Not From Concentrate (NFC) juice production with emphasis on increasing juice quality and expanding seasonal availability. Improved clones from seedling introductions, budwood irradiation and somaclonal variation will be discussed. The fresh fruit industry in Florida is dominated by grapefruit and seedy mandarins. Primary objectives for fresh fruit improvement include expanding seasonal availability, development of canker tolerant and low furanocoumarin grapefruit, and development of seedless, easy-peel mandarins. The focus of our program has been on parent development, and seedless triploid production via interploid crosses; strategies and successes will be discussed. Rootstock Improvement: Initial focus was on improving soil adaptation, tree-size control to facilitate harvesting and cold-protection, and disease resistance, especially to blight, CTV-induced quick decline and Phytophthora. Breeding objectives have since been expanded to include tolerance of the Diaprepes/Phytophthora complex and Huanglongbing HLB, and development of rootstocks that facilitate advanced citrus production systems (ACPS). Progress from conventional breeding (both 2x and 4x) and somatic hybridization will be discussed. Genetic Engineering: Due to the HLB and canker epidemics, focus has been on disease resistance gene discovery, transformation, and evaluation of transgenic plants; progress will be discussed.

DIFFERENCES IN THE GENETIC STRUCTURE OF CITRUS TRIPLOID HYBRIDS RECOVERED FROM 2X × 2X AND 4X × 2X SEXUAL HYBRIDIZATIONS

Citrus triploid hybrids can be recovered by 2x X 2x hybridisations as a consequence of the formation of 2n gametes or by interploid hybridisation. Most of the 4x parents used in our program are doubled-diploid (DD), also known as autotetraploid. The genetic structure of diploid gametes and particularly the rate of parental heterozygosity restitution (PHR) depend on the meiotic process by which they were originated. Second-division restitution (SDR) is the 2n gamete formation mechanism involved in 'Fortune' and clementines. With SDR, PHR is positively linked with the distance of each locus to the centromere. With DD parents, PHR depends on the rate of preferential pairing and thus the proportion of disomic versus tetrasomic segregations. We have compared the genetic structure of two populations of diploid gametes of clementine (one population of 2n gametes and one of 2x gametes produced by DD) with SNP and SSR markers covering the 9 citrus chromosomes. The DD displays mostly tetrasomic segregation; however three linkage groups present intermediate segregation and one displays a tendency for disomy. The PHR in the 2n gametes is 2/3 of that obtained in 2x gametes produced by DD. The two methods of triploid production appear complementary in terms of genotypic variability. 4x X 2x hybridizations are more efficient than 2x X 2x for developing new cultivars phenotypically closer to the diploid parent of the DD. Conversely, 2x X 2x hybridisations provides a greater opportunity to select innovative products. (Resume)

Ploidy Levels and Pollen Stainability of Lantana camara Cultivars and Breeding Lines

Lantana camara L., a popular nursery and landscape plant, is categorized as an invasive species in Florida, because it produces viable pollen and cross-pollinates with the native species Lantana depressa Small. The invasive potential of L. camara is a challenging issue for the nursery and landscape industry, so sterile non-invasive cultivars are needed to replace fertile invasive ones. This study aimed to determine the ploidy level and male fertility of both commercial L. camara cultivars and breeding lines to identify male-sterile cultivars and assess the effectiveness of sterile triploid production in L. camara. A polyploid series was identified among 32 L. camara cultivars and breeding lines. Male fertility, based on pollen stainability, varied widely among the cultivars/breeding lines. Ploidy level was the most important factor determining L. camara pollen stainability/male sterility. On average, diploids exhibited the highest pollen stainability (64.6%) followed by tetraploids (45.1%), pentaploids (34.6%), and hexaploids (18.0%). Triploids showed the lowest pollen stainability (9.3%), suggesting that generating triploids would be an effective genetic approach to producing sterile L. camara and reducing its pollen-mediated invasiveness. Pollen stainability of triploid cultivars, Balandpawn (LandmarkTM Pink Dawn PP15,516), Lemon Drop, Miss Huff, New Gold, New Red Lantana, Red Butler, Red Spread Lantana, Samson Lantana, and Sunset Lantana was consistently below 10%. A number of triploid cultivars had pollen stainability approaching 20% to 30%, indicating a necessity for careful examination and screening of newly produced triploids to ensure high sterility in selected triploids. Pollen stainability variation was observed within ploidy levels, implying the existence of other genetic and environmental factors that influence the pollen stainability/male fertility of L. camara. Results from this study suggest that there is excellent potential to develop genetically sterile cultivars in L. camara for the U.S. nursery and landscape industry.

Assignment of parentage in triploid species using microsatellite markers with null alleles, an example from Pacific oysters (Crassostrea gigas)

Triploid production in aquaculture is increasing because of their more profitable growth and reproduction traits. Triploids are mostly produced through mass spawning techniques, meaning that exact pedigree is unknown. The ability to trace the pedigree of high performing triploids would allow selection of broodstock to perpetuate triploids of greater economic value. This study aimed to develop a method of determining parental assignment in triploids and test its accuracy on triploid oysters. Using a likelihood approach and accounting for null allele frequencies, a method was developed which proved to be efficient at determining the pedigree of triploid oysters. This method was able to provide accurate pedigree on simulated data and two commercial cohorts of triploid oysters. The analysis of the triploid cohorts showed that mass spawning to produce triploid oysters, like that for diploid and tetraploids, results in a strong bias in parental contributions, with the effective population size being 34-49% lower than the census population. This highlights the need for pedigree control in breeding programs and indicates that the ability to determine parentage of triploids will be a valuable tool for breeding programs.

The autotetraploid fish derived from hybridization of Carassius auratus red var. (female) × Megalobrama amblycephala (male).

The establishment of the tetraploid organism is difficult but useful in genetics and breeding. In the present study, we have artificially established an autotetraploid fish line (F2-F8) derived from the distant hybridization of Carassius auratus red var. (RR, 2n = 100) (female) × Megalobrama amblycephala (BB, 2n = 48) (male). The autotetraploid line (F2-F8) possess four sets of chromosomes from red crucian carp (RRRR, 4n = 200) and produce diploid ova and diploid sperm, which maintains the formation of the autotetraploid line. The F2 of the autotetraploid fish result from the fertilization of the autodiploidy diploid eggs and diploid sperm from the females and males of F1 hybrids (RRBB, 4n = 148), which exhibit abnormal chromosome behavior during meiosis as revealed by gynogenesis and backcrossing. This is the first report concerning the establishment of an autotetraploid fish line derived from distant hybridization. The autotetraploid fish line provides an important gamete source for the production of triploids and tetraploids. The autotetraploid fish line also provides an ideal system to investigate the poorly understood mechanisms that drive diploidization in autotetraploids and to study the hybrid progenies' characteristics, including the appearance of new traits that promote a diversity of traits and facilitate adaptation.

Megasporogenesis, megagametogenesis, and induction of 2n eggs with colchicine in poplar section Aigeiros

Triploid breeding through 2n eggs induction has become one of the most powerful approaches for the improvement of the genus Populus. To enhance the efficiency of triploid production in section Aigeiros, female buds (catkins) of P. × euramericana (Dode) Guinier at different developmental stages were exposed to colchicine for 2n eggs induction. The relationship between flower bud morphological characteristics (or time after pollination) and female meiotic stage (or embryo sac development) was established to guide colchicine treatments. In the resulting progeny, three triploid seedlings were obtained via embryo rescue and identified by flow cytometric analyses. Cytological observation revealed that the leptotene to pachytene stage of meiosis and 12 h after pollination might be the suitable period for 2n eggs induction with colchicine in P. × euramericana. Results and protocols related to 2n eggs induction, effective methodologies for embryo rescue, and ploidy evaluation in this study might be applicable in polyploidy breeding in section Aigeiros.

Heteroploid mosaic tetraploids of Crassostrea virginica produce normal triploid larvae and juveniles as revealed by flow cytometry

Abstract Advantages associated with sterility in triploids have made them the standard for commercial aquaculture of Crassostrea virginica in the Chesapeake Bay. Current hatchery practices for commercial production of triploids employ tetraploids as broodstock, and thus tetraploids are a core asset for oyster culture. Tetraploids undergo reversion, losing entire sets of chromosomes, and become heteroploid mosaics comprised of triploid and tetraploid cells. The possible effects of using mosaic tetraploids for triploid production are a practical concern for commercial oyster culture. We crossed mosaic and non-mosaic tetraploids, males and females, to a reference diploid oyster and compared relative DNA content and phenotypic qualities of triploid progeny. Relative DNA content was measured in somatic and gametic tissue of tetraploid broodstock, as well as in resulting triploid larvae via flow cytometry. Size, abnormality, and survival of larvae were recorded through the rearing process. For six crosses, triploids were raised for 1 year and shell metrics, size, and somatic DNA content were compared. Flow cytometry analysis of broodstock revealed sperm from mosaic tetraploids were consistently di-haploid and virtually indistinguishable from sperm of non-mosaics. For resulting triploid progeny, there were no detectable differences between larvae or juveniles produced from mosaic and non-mosaic tetraploids. Differences existed, however, between triploid larvae produced from di-haploid sperm and those produced from tetraploid eggs: triploids from tetraploid eggs were significantly more abnormal, less viable, and grew slower. Our findings support current commercial practices of using tetraploid males as the principal parent for triploid production and demonstrate no practical consequence from using mosaic tetraploid broodstock for commercial production of triploid C. virginica . Further analysis via cytogenetic techniques may provide insight into long term consequences of using mosaic tetraploids, for example, in tetraploid × tetraploid crosses.

Comparison of disease resistance between diploid, induced-triploid, and intercross-triploid rainbow trout including trout selected for resistance to Flavobacterium psychrophilum

All-female triploid fish are advantageous in rainbow trout (Oncorhynchus mykiss) aquaculture due to sterility and the consequent superior filet quality, growth, and feed conversion achieved at reproductive size. Triploid fish are commonly produced by pressure or temperature shock of the zygote (induced-triploids, 3NP), but can also be produced by mating a tetraploid parent with a diploid (2N) parent (intercross-triploids, 3NC). Little is known about the disease resistance of 3NC fish. In this study, we measured post-challenge survival of genetically-related 2N, 3NP and 3NC families after exposure to Flavobacterium psychrophilum the etiological agent of bacterial cold water disease (BCWD). The families used in this study originated from either an unselected commercial stock or BCWD resistant or susceptible line of rainbow trout. The overall survival of 2N fish was slightly higher than the respective 3NP and 3NC fish. Although 3NC fish tended to show higher survival than 3NP fish, this response was not sufficiently consistent to promote intercross production of triploids for increasing disease resistance. The survival of ten sets of 3NP, 3NC and 2N families, created from BCWD resistant and susceptible line parents, demonstrated overall similar inherent relative survival differences among the ploidy types both among families and between lines, although family differences in response to triploidization and triploidization strategy were observed. In summary, minimal survival difference between 3NP and 3NC families following experimental challenge suggests that innate resistance to F. psychrophilum is not substantially altered by the triploidization strategy and progress in family-based selective breeding of diploid fish for BCWD resistance will substantially translate into improved survival when triploids are derived either by shock treatment or intercross breeding.

Confirmation of cytotype stability in autotetraploid black wattle (Acacia mearnsii) trees using flow cytometry and size differences of the reproductive gametes

Acacia mearnsii (black wattle) is grown commercially in South Africa for its timber and bark. However, the invasive nature of the species has resulted in it being considered an alien invader and for this reason research has been aimed at producing a sterile triploid variety that would be highly desirable for the South African commercial forestry industry. Tetraploids were successfully induced by soaking germinating diploid seeds in colchicine. Seed from these tetraploids was used to establish a field trial, where crossing diploid with tetraploid parent plants only produced diploid and tetraploid progeny and failed to produce any triploid progeny. Control-crossed seed set between diploids is generally low in A. mearnsii and, together with the possibility of an unstable tetraploid population, this could be reducing the chances of producing triploid seed. Thus identification and confirmation of stability within the existing advanced-generation tetraploid population (aged 10–11 years) was critical to ensure the production of sterile triploids. Flow cytometry was used to determine the stability of the ploidy of leaf vegetative tissues, whereas polyad and ovule size measurements were used to determine the stability of ploidy of the reproductive tissues. Results from the study revealed that the tetraploidy of within the leaf vegetative tissue was stable. For both the ovule and polyad size measurements, a size range was determined for diploids and tetraploids and, within the population under investigation, no overlap was apparent. This allowed for the conclusion that the advanced-generation tetraploid population was stable and that the absence of triploid progeny must be because of post-zygotic reproductive barriers within the ovary.

Induction of 2n female gametes in Populus adenopoda Maxim by high temperature exposure during female gametophyte development.

In order to produce triploid plants, 2n female gametes were induced by treating female buds and developing embryo sacs of Populus adenopoda Maxim with high temperature exposure. During megasporogenesis, tests were conducted on the relationship between female gametophyte development and morphological changes of female catkins. In the resulting progeny, 12 triploids were produced, and the highest rate of triploid production was 40%. Cytological observation revealed that the pachytene to diakinesis phase of meiotic stages may be a suitable period for inducing megaspore chromosome doubling through high temperature exposure. On the other hand, catkins of 6–72 h after pollination were treated for inducing embryo sac chromosome doubling. In the offspring seedlings, 51 triploids were detected and the highest efficiency of triploid production was 83.33%. Correlation analysis between the proportion of each embryo sac’s developmental stage and the percentage of triploid production indicated that the second mitotic division may be the most effective stage for 2n female gamete induction. Our findings showed that high temperature exposure is an ideal method for 2n female gamete induction. Heterozygous offspring are valuable for breeding programs of P. adenopoda.

High temperature-induced triploid production during embryo sac development in Populus

Abstract Triploid breeding plays an important role in cultivar improvement in the genus Populus L. A novel approach for triploid production with colchicine during embryo sac development was reported recently by Wang et al. (2010). In the present investigation, female catkins of Populus pseudo-simonii × P. nigra ‘Zheyin3#’ during embryo sac development were exposed to high temperature to assess the effectiveness of high temperature for induction of triploid production. In the progeny, 45 triploids were determined by both flow cytometric analysis and somatic chromosome counting. The period 66-72 h after pollination was the most suitable for high temperature-induced triploid production during embryo sac development in the ‘Zheyin3#’. Cytological analysis showed that the frequency of eight-nucleate embryo sacs rose at an increased rate during 66-78 h after pollination, which suggested that the third mitosis during embryo sac development could be the optimal stage for high temperature-induced triploid production. The highest frequency of triploid production was 40%, which was obtained in the 44°C for 2 h treatment 72 h after pollination. In view of both triploid number and production efficiency, treatments with 41°C for 4-6 h or with 44°C for 2 h during 66-72 h after pollination were both effective for triploid induction in ‘Zheyin3#’. Statistical analysis showed that the growth of triploids and diploids was not significantly different. However, highly significant differences were observed for all leaf characteristics. Finally, the significance of high temperature treatment in Populus triploid breeding programs is discussed.

Hormonal sex reversal in Atlantic cod, Gadus morhua

Pre-harvest sexual maturation, a major constraint to aquaculture operations, commonly occurs in both sexes of Atlantic cod (Gadus morhua). Production of all-female triploids would alleviate this problem because they are sterile. The objective of this project was to develop effective endocrine manipulations to produce monosex stocks of cod, as the first step towards producing all-female triploids, using dietary 17β-estradiol (E2) and 17α-methyldihydrotestosterone (MDHT) fed during the labile period of gonadal differentiation. In the first experiment, triplicate treatments of E2 at 5, 10 and 20mg/kg feed and MDHT at 0.67, 2 and 6mg/kg feed were fed to fish during the growth interval from 17±2 to 43±4mm standard length (SL). The only treatment which affected sex ratio was the highest MDHT dose, with a significant reduction in proportion of females compared to controls. Based on these results and a thorough histological examination of gonadal development, a follow-up experiment used duplicate treatments of E2 at 20, 40, 80 and 120mg/kg and MDHT at 3, 6, 12 and 18mg/kg fed to fish during the growth interval from 7.8±1.2 to 45±2mm SL. This resulted in a dose-dependent shift in sex ratio in both estrogen and androgen treated groups, with no females observed in the groups fed the two highest doses of MDHT and a significantly reduced male ratio in E2 groups at the two highest doses. These results indicate that successful sex reversal can be achieved through dietary hormonal manipulation when the treatment starts at around 8mm SL.

Triploid Oysters in the Chesapeake Bay: Comparison of Diploid and TriploidCrassostrea virginica

Diploid and triploid Eastern oysters, Crassostrea virginica, were tested at 3 sites characterized by low or moderate salinity regimes in the Virginia part of the Chesapeake Bay from November 2005 through October 2007. Both diploid and triploid cultures were replicated 3 times by producing separate spawns from different broodstock. Ploidy had a generally consistent effect on the performance of C. virginica at the 3 test sites. At the end of the study, in October 2007, and across all sites, triploid oysters had lower cumulative mortality than diploids (-34%), and greater shell height ( 25%), whole weight ( 88%), and yield ( 152%). as well as a higher proportion of market-size oysters ( 114%) than diploids. Both diploids and triploids were similarly infected by Perkinsus marinus and, to a lesser extent, by Haplosporidium nelsoni. In a closer look, growth parameters (shell height growth, whole weight, yield, and percentage of marketable oysters) were always higher in triploids than in diploids regardless of the parental source, strongly supporting the superior advantage of triploids. Similar results were obtained for cumulative mortality, but to a lesser extent as a result of the large variation in mortality for both diploid and triploid cohorts among sites, as well as a significant site-by-cohort interaction. Our report is the first clear illustration of variation for the cumulative mortality exhibited among different spawns in triploids, and comes with the lesson that care must be taken in experiments in which the goal is to test the effect of ploidy on this trait. Our results support the notion that selective breeding programs to reduce mortality, coupled with triploid production to increase growth, can further optimize yield. The best-performing replicate spawn had 80% survival after 2.5 y, and reached an average shell height of 92 mm, weighing 142 g.

Induction of unreduced megaspores with high temperature during megasporogenesis in Populus

Triploid breeding is one of the most powerful approaches for improvement of the genus Populus L. Pollination with artificial unreduced (2n) pollen was inefficiency, owing to weak competition of 2n pollen. To induce 2n megaspores and improve the efficiency of triploid production, female buds of Populus pseudo-simonii × Populus nigra ‘Zheyin3#’ were exposed to high temperature during megasporogenesis. A relationship between megasporogenesis and morphological changes of female buds was established to guide the high-temperature treatments. In the progeny, 146 triploids were obtained and the highest efficiency of triploid production was 66.7%. Both 41°C and 44°C were suitable for megaspore chromosome doubling. Cytological analysis showed that meiotic stages from pachytene to diplotene may be the optimal period for megaspore chromosome doubling at high temperature in the ‘Zheyin3#.’ Because megasporocytes both in the first meiotic division and the second division were treated, the first meiotic division restitution typed and the second meiotic division restitution typed 2n megaspores could be obtained. Our findings indicated that hybridization with high-temperature-induced 2n megaspores is an ideal approach for triploid production. Offspring with different heterozygosity are valuable for genetic research and breeding programs of Populus.

Massive production of all-female diploids and triploids in the crucian carp.

In many species of aquaculture importance, all-female and sterile populations possess superior productivity due to faster growth and a relatively homogenous size of individuals. However, the production of all-female and sterile fish in a large scale for aquaculture is a challenge in practice, because treatments necessary for gynogenesis induction usually cause massive embryonic and larval mortality, and the number of induced gynogens is too small for their direct use in aquaculture. Here we report the massive production of all-female triploid crucian carp by combining artificial gynogenesis, sex reversal and diploid-tetraploid hybridization. Previously, we have obtained an allotetraploid carp population (4n = 200) by hybridization between red crucian carp (Carassius auratus red var; ♀) and common carp (Cyprinus carpio; ♂). We induced all-female diploid gynogens of the Japanese crucian carp (Carassius cuvieri; 2n = 100). We also generated male diploid gynogens of the same species treated gynogenetic fry with 17-α-methyltestosterone, leading to the production of sex-revered gynogenetic males. Finally, these males were used to cross with the female diploid Japanese crucian carp gynogens and the allotetraploid females, resulting in the production of fertile all-female diploid Japanese crucian carp (2n=100) and sterile all-female triploid hybrids (3n = 150), respectively. Therefore, diploid crucian carp gynogenetic females and sex-reversed male together with an allotetraploid line provide an opportunity to produce all-female triploid populations in a large scale to meet demands in aquaculture industry.

Protoplast fusion for production of tetraploids and triploids: applications for scion and rootstock breeding in citrus

Protoplast fusion technology has been utilized in many crops to generate allotetraploid somatic hybrids, and sometimes autotetraploids as a byproduct of the process. A brief history of this technology development is provided, along with a simple protocol developed for citrus, which can be easily adapted to other plants. Protoplast fusion has become a significant tool in ploidy manipulation that can be applied in various cultivar improvement schemes. In rare cases, a new somatic hybrid may have direct utility as an improved cultivar; however, the most important application of somatic hybridization is the building of novel germplasm as a source of elite breeding parents for various types of conventional crosses for both scion and rootstock improvement. Somatic hybridization is generating superior allotetraploid breeding parents for use in interploid crosses to generate seedless triploids. Seedlessness is a primary breeding objective for new fresh fruit citrus varieties, and several thousand triploid hybrids have been produced using somatic hybrids as the tetraploid parent. Protoplast fusion is also being utilized to produce somatic hybrids that combine complementary diploid rootstocks, which have shown good potential for tree size control. Tree size control has gained importance as a means of reducing harvesting costs, maximizing the efficiency of modern cold protection methodology, and facilitating the adaptation of new fruit production systems. Successful somatic hybridization in citrus rootstock improvement has enabled rootstock breeding at the tetraploid level via sexual hybridization, which can yield maximum genetic diversity in zygotic progeny upon which to impose selection for the many traits required in improved rootstock cultivars, including disease and insect resistance, broad adaptation, tree size control, and the ability to consistently produce high yields of quality fruit. Recent progress and successful examples of these applications are discussed. Finally, a discussion of the genetic potential of somatic hybrids as breeding parents, including meiotic behavior and inheritance is provided.

Mitochondrial DNA Dosage Effects in Triploid Grass Carp

AbstractThe grass carp Ctenopharyngodon idella is used widely in aquaculture for aquatic weed control. Historically, concern about the proliferative growth of nonnative diploid grass carp populations led to development of the hydrostatic pressure shock technique for the production of sterile triploids. Despite the success of this technique in accomplishing management goals, little is known about the cellular physiology of triploids. In particular, few studies have explored whether the induction of triploidy via hydrostatic pressure shock affects mitochondrial DNA (mtDNA) copy number in addition to that of chromosomal DNA. While the nuclear DNA (nDNA): mtDNA ratio is conserved in polyploid yeast, there are few data suggesting that vertebrate polyploids have nDNA: mtDNA ratios consistent with those of their diploid counterparts. In this study, two novel quantitative polymerase chain reaction assays were used to explore gene copy number in the nuclear and mitochondrial genome of diploid and triploid grass carp and to determine whether nuclear–mitochondrial copy number differences can be exploited to determine the ploidy of individual fish. The results of this study suggest that the ratio of mtDNA to overall cellular DNA is conserved in vertebrate polyploids and that real‐time assays of mtDNA copy number are not a feasible alternative to current methodologies for identifying triploid grass carp.

Use and exchange of aquatic genetic resources in aquaculture: information relevant to access and benefit sharing

Use and exchange of aquatic genetic resources in aquaculture: information relevant to access and benefit sharing