Masaru Iwanaga

Transfer of resistance to PLRV titer buildup from Solanum etuberosum to a tuber-bearing solanum gene pool

SummaryHybrids between Solanum etuberosum and S. pinnatisectum harboring resistance to titer buildup of potato leafroll virus (PLRV) were reciprocally crossed with tuber-bearing wild species S. acaule and S. verrucosum. A total of 47 hybrids with acaule were obtained with the aid of embryo rescue and sterile culturing of embryos from imbibing seeds. All but two hybrids with acaule had low pollen stainabilities or were pollen sterile. Hybrid seeds from crosses with verrucosum were easily obtained, and the triploid progenies were sterile. Hybrid progeny were screened for resistance to PLRV infection by viruliferous green peach aphid and for resistance to titer buildup. Although hybrids did not exhibit resistance to infection, PLRV was not detectable using ELISA. Virus was detected, however, by graft transmission to Datura stramonium. Crosses of fertile acaule-etuberosum-pinnatisectum hybrids with S. phureja, a cultivated diploid, using the latter as pollen parent, produced berries but seed did not complete development and was aborted. Rescue of immature embryos 25 days after pollination by excision from berries and sterile culture produced vigorous, pot-cultured plants. Segregation of susceptible (virus detected) and resistant (virus undetected) progenies suggests simple inheritance.

A restorer gene for genetic-cytoplasmic male sterility in cultivated potatoes

Fifty-nine clones (cultivars and advanced selections) ofSolanum tuberosum Group Tuberosum were pollinated with two Group Phureja-haploid Tuberosum hybrids, which produce FDR 2n pollen (ps/ps) and are heterozygous for a dominant gene (Ms/ms) which interacts with Tuberosum cytoplasm to result in male sterility. Twenty-eight and thirty-four plants from each family were transplanted to the field in two different locations, respectively. Pollen samples were collected from each plant and stained with acetocarmine glycerol jelly. Plants with over 95% unstainable pollen were considered male sterile. The families could be put into distinct classes according to the ratio of male sterile to male fertile plants; 3 sterile: 1 fertile (23 clones), 2 sterile: 3 fertile (24 clones), 1 sterile: 5 fertile (3 clones), and 0 sterile: 1 fertile (3 clones). Four families deviated significantly from any expected ratio. The variation for male fertility among families resides in the tetraploid parent, since the male parents have the same genotype (Ms/ms) for theMs locus. The locus of this gene is very distal to the centromere such the 2n gametic output is 1 Ms/Ms : 2 Ms/ms : 1 ms/ms. The variation in male sterility is explained by the presence or absence of a dominant allele of a restorer gene (Rt) in tetraploid Tuberosum. This locus is also very distal to the centromere. The frequency of theRt gene was estimated to be 0.20 in Tuberosum. The finding ofRt gene explains some of the conflicting results in male fertility of haploid Tuberosum x 2x cultivated and wild species hybrids. It also provides an opportunity to partially circumvent this male sterility. Haploids (2n = 2x = 24) extracted from 4x clones withRt genes would have male fertile progeny in crosses with species carrying theMs gene.CompendioCincuenta y nueve clones (cultivares y selecciones avanzadas) deSolanum tuberosum Grupo Tuberosum fueron polinizadas con dos hibridos entre Grupo Phureja - haploides de Tuberosum, que producen polen 2n por restitucion nuclear de la primera division (ps/ps), y son heterozigotes para el gen dominante (Ms/ms), que interactua con el citoplasma de Tuberosum produciendo esterilidad masculina. Veinte y ocho y treinta y cuatro plantas de cada familia fueron transplantadas al campo en dos diferentes localidades, respectivamente. Muestras de polen fueron colectadas en cada planta individualmente y tenidas con acetocarmin glicerol jelly. Plantas con menos de 5% de polen tenido fueron consideradas macho-esteriles. Las familias pudieron ser agrupadas de acuerdo a la proporcion de plantas machoesteriles a macho-fertiles; 3 esteriles : 1 fertil (23 clones), 2 esteriles : 3 fertiles (24 clones), 1 esteril : 5 fertiles (3 clones), y 0 esteril : 1 fertil (3 clones). Cuatro familias se desviaron significativamente de la proporcion esperada. La variacion de fertilidad masculina entre las familias se debe al progenitor tetraploide, ya que los progenitores masculinos tienen el mismo genotipo (Ms/ms) para el locusMs. El locus de este gen esta ubicado muy lejos del centromero, de tal modo que la produccion de gametos 2n es 1 Ms/Ms : 2 Ms/ms : 1 ms/ms. La variacion en esterilidad masculina es explicada por la presencia de un gen dominante restaurador de fertilidad (Rt) presente en el progenitor tetraploide Tuberosum. La frecuencia del genRt fue estimada en 0.20 en Tuberosum. El hallazgo del genRt explica algunos de los resultados conflictivos en esterilidad masculina de hibridos entre haploides de Tuberosum y especies cultivadas y silvestres. Asimismo, provee una oportunidad para parcialmente evitar esta esterilidad masculina. Haploides (2n = 2x = 24) extraidos de clones 4x con genesRt tendrian progenies macho-fertiles en cruzamientos con especies que poseen el genMs.

Diploid potato germplasm derived from wild and land race genetic resources

Diploid cultivated species, haploids derived from tetraploid cultivars, and their wildSolanum relatives were used to generate various diploid potato genetic stocks. These stocks have significant value in potato breeding because: 1) genetic diversity in the potato population can be kept variable with the diploid germplasm which confers genetic variation from wild and cultivated species; 2) crossability of this diploid germplasm is facilitated by the function of 2n gametes; and 3) high levels of pest resistances originating from the process of introgression of the wild and cultivated genetic resources can be used to improve the tetraploid cultivated potato genepool.

Breaking the crossability barriers between disomic tetraploid Solanum acaule and tetrasomic tetraploid S. tuberosum

SummaryA combination of compatible second pollinations and embryo rescue was applied for systematic production of true tetraploid hybrids from crosses between disomic tetraploid Solanum acaule and tetrasomic tetraploid potato, S. tuberosum. Several genotypes of tetraploid potatoes were pollinated with S. acaule, and the compatible second pollinations were made on the following day, with a genotype of S. phureja, IvP 35 to promote fruit development. Embryo rescue was carried out in 21 families, 14 to 27 days after the first pollination. A total of eight plants were obtained from the embryo rescue and their chromosome numbers were counted in the root tips. Three of the eight plants were identified as tetraploid, and five others as diploid. Morphology, isozyme banding patterns, and pollen stainability, as well as potato spindle tuber viroid (PSTVd) resistance, indicated the hybrid nature of the three plants. This is the first report of successful tetraploid hybrid production between disomic tetraploid S. acaule (4x) and tetrasomic tetraploid potatoes. Seed set from the crosses between one of hybrids and diploid potatoes indicated workable levels of both male and female fertility for introgression of valuable genes from S. acaule into the cultivated potato gene pool. The methodology used may be applied to other disomic tetraploid tuber-bearing Solanum species and with some modifications also to distantly related solanaceous species and genera.

Discovery of a synaptic mutant in potato haploids and its usefulness for potato breeding.

SummaryA synaptic mutant was found in haploids (2n=2x=24) extracted from the Mexican potato variety ‘Atzimba’ (2n=4x=48). The mutant is inherited as a simple Mendelian recessive, designated sy4. Meiotic abnormalities of the mutant during microsporogenesis include: poor synapsis at pachytene; high frequency of univalents at diakinesis; elongated and curved spindles and univalents being scattered over the spindles at metaphase I and anaphase I; abnormal chromosome distribution at anaphase I; and production of sterile pollen, presumably due to unbalanced chromosome complement. The expression of sy4 in megasporogenesis was also detected. The sy4 mutant is very useful for potato breeding when combined with another meiotic mutant, parallel spindles (ps), because haploids homozygous for sy4 and ps produce fertile 2n pollen which transmit almost intact genotypes of the parents to the progenies. Thus, the meiotic mutants provide a powerful breeding method for maximizing heterozygosity and epistasis. They can also provide a very efficient method of transferring diploid germplasm, which has desired characteristics efficiently combined at the 2x level, to tetraploids. Many haploids have been identified with 2n pollen production by ps alone or by sy4 and ps, vigorous growth and good flowering, and a high level of resistance to late blight. The importance of a further search for meiotic mutants and their use for breeding is discussed.

Transmission of bacterial wilt resistance by First Division Restitution (FDR) 2n pollen via 4x×2x crosses in potatoes

SummaryResistance to bacterial wilt (Pseudomonas solanacearum) found in tuber-bearing Solanum species was transferred into a diploid potato breeding population. Simultaneous selections were made for agronomic characters, production of first division restitution (FDR) 2n pollen, and resistance. Diploid resistant genotypes were identified via inoculation with a virulent isolate (CIP-204) of race 3 of Pseudomonas solanacearum. These resistant diploid genotypes were crossed to susceptible tetraploid potatoes. An investigation was made to assess whether resistant diploid genotypes transmit resistance to bacterial wilt, which is a quantitatively inherited trait, to tetraploid potatoes via FDR 2n pollen. Tetraploid seedlings from 4x×2x crosses were inoculated with the same isolate CIP-204, and the percentage of surviving seedlings was scored. Some 4x×2x families from resistant diploid genotypes demonstrated a high level of survival rate. The transmission of bacterial wilt resistance was achieved by the use of FDR 2n pollen. It was speculated that a female x male interaction effect on the survival rate of the evaluated progeny may exist. Selecting proper 4x and 2x parents would be important for obtaining a higher frequency of transmission of resistance to bacterial wilt in the progeny.

Genetic control of embryo formation in anther culture of diploid potatoes

A diploid potato clone AH 78/8015.37a with androgenetic ability was crossed with a root-knot nematode resistant diploid clone 381320.23 which did not have this ability.Among 19 F1 progenies tested a wide range of continuous variability was found for androgenetic capability. Four F1 clones with different level of embryo formation capability were backcrossed to 381320.23 to produce 4 F1BC1 families to further clarify the genetic control of androgenetic capability.From the wide range of continuous variability for androgenetic ability observed, it can be inferred that this character is controlled by more than one major gene.The occurrence of plants with androgenetic ability derived from parents both lacking this character, indicated that the androgenetic ability is controlled by recessive genes or results from complementation of various factors singly present in the parents. The present results demonstrated that the androgenetic ability could be transferred through sexual crosses, making it possible to successfully apply anther culture to other useful genetic material lacking this ability.

Efficiency of potato breeding using FDR 2n gametes for multitrait selection and progeny testing

SummaryThe objective of this research was to compare the efficiency of the 4x×2x breeding scheme with the traditional 4x×4x method with respect to potato improvement. The basis for such a comparison was the parental value of four 2x and four 4x male parents from the International Potato Center (CIP) as measured by multitrait selection and progeny testing. The 2x parents produced 2n pollen by parallel spindles at anaphase II, which is genetically equivalent to a first division restitution (FDR) mechanism. Both 2x and 4x parents were crossed with four common 4x female parents. Thus, 32 families were evaluated over 2 years at four Peruvian locations. A selection index which considered tuber yield, tuber number, average tuber weight and specific gravity was used for multitrait selection. Three FDR 2x parents had better selection index scores than the 4x parents over the four locations. Estimates of broad-sense heritability for total yield using different number of replications and locations were calculated by using the variance components. The 4x × 2x breeding scheme was found to be better than the traditional 4x × 4x method since fewer replications and locations are required to evaluate tuber yield in 4x × 2x progenies than in 4x × 4x progenies. The FDR 2x parents were also better material than the 4x parents for testing combining ability for tuber yield of the 4x progenitors. This could be the result of the mode of FDR 2n pollen formation. The pollen of FDR 2x parents is more heterozygous, but more homogenous than n pollen from 4x parents.

Application of interspecific sesquiploidy to introgression of PLRV resistance from non-tuber-bearing Solanum etuberosum to cultivated potato germplasm

SummaryHybridization of synthetic allotetraploids of S. pinnatisectum with S. etuberosum (4x-EP) with S. acaule (2n = 4x = 48) resulted in two individuals that were highly fertile, in contrast to all other progenies. The unique individuals are hexaploids, 2n = 72, while the other progenies are tetraploids, 2n = 48. They are thought to be the products of a union between 2n eggs of S. acaule and normal 1n microspores of 4x-EP. The fertile hexaploids (designated 6x-AEP) produced abundant selfed seed and viable hybrids with cultivated diploid potato, S. phureja, when developing embryos were rescued from berries and cultured before transplanting to pot culture. The extreme variability in chromosome constitution of the hybrids with S. phureja and selfed progenies indicates that addition and substitution lines of etb chromosomes bearing genes of interest to breeders could easily be produced from this material. The production of sesquiploids, as the 6x-AEP hybrids are called, is discussed as a useful bridging step in the introduction of alien genes from genomes that share little homology with the cultivated genome.

The inheritance of β-amylase null in storage roots of sweet potato,Ipomoea batatas (L.) Lam.

SummarySeveral sweet potato genotypes were found to lack completely or to have only traces ofβ-amylase in their storage roots. Such genotypes do not increase in sweetness during cooking because, without a sufficient amount ofβ-amylase, the normal hydrolysis of starch to maltose does not occur in the cooking process. In order to study the inheritance of this biochemical variant in the genotype, 41 families were generated. The following conclusions were drawn from analyzing these families. (1) This trait is controlled by one recessive allele (designatedβ-amy) (2) It is inherited in a hexasomic or tetradisomic manner, but not disomically or tetrasomically. This conclusion supports previous cytological data that sweet potato is an autohexaploid or has two identical genomes plus one genome which is somewhat different. (3) Theβ-amy allele appears to exist at a high frequency in cultivated germplasm. (4) Breeding sweet potato for lowβ-amylase activity is relatively easy. New types of sweet potato without normalβ-amylase activity have great potential for processing and as a staple food.