Plant Breeding Reviews

By Jules Janick

John Wiley & Sons

Copyright © 2005 John Wiley & Sons, Inc.
All right reserved.

ISBN: 0-471-66693-9

Chapter One

Dedication: Stanley J. Peloquin Potato Geneticist and Cytogeneticist

Rodomiro Ortiz* International Institute of Tropical Agriculture (IITA), Nigeria; IITA c/o Lambourn Ltd. (UK), 26 Dingwall Road, Croydon, CR9 3EE, UK

Luigi Frusciante and Domenico Carputo Departmento of Soil, Plant and Environmental Sciences, University of Naples "Federico II", Via UniversitÓ, 100, 80055 Portici, Italy

Dr. Stanley J. Peloquin is known to plant breeders for his decisive contributions to genetic enhancement of potato (Solanum tuberosum L.) using haploids, 2n gametes, and wild Solanum species; for his pioneering work on potato cultivation through true seed; and as mentor of a new generation of plant breeders worldwide. The genetic enhancement of potato, the fourth most important food crop worldwide, benefited significantly from Peloquin's work on ploidy manipulations led by the genetic knowledge he and his co-workers, mostly former graduate students, created and systematically transformed into applied breeding methods. His scientific papers, book chapters, classes, seminars, and talks on potato as a model plant for genetic breeding and evolutionary research are a source of inspiration to all researchers in crop improvement.


Stan was born in Barron, Wisconsin, in 1921. He went to grade school in three small towns in Wisconsin and to a small high school at Ondossagon near Lake Superior in northern Wisconsin, where he was taught to get out of northern Wisconsin to make a living. There were 20 in his graduating class. After graduating with a degree in chemistry in 1942 from River Falls State College (now the University of Wisconsin-River Falls), he joined the U.S. Navy and served on a destroyer in the South Pacific for 3-1/2 years during the Second World War. Upon his release from the service, he enrolled at Marquette University, Milwaukee, Wisconsin, and obtained a MSc degree in biology in 1948. He then enrolled in the University of Wisconsin and studied genetics under the guidance of Dr. R. A. Brink and Dr. D. C. Cooper and was awarded a PhD degree in genetics in 1952. His thesis was entitled "Abnormal Embryo and Endosperm Development in Zea mays Following the Use of Pollen that has been Exposed to Mustard Gas." From 1951 to 1956, he taught biology at Marquette University. From 1957, when he joined the faculty at the University of Wisconsin, Madison, until his retirement in 1994, he taught genetics to undergraduates in the Biocore Biology Program, and two graduate level courses: cytogenetics, and, with Dr. Ted Brigham, Chromosome Manipulations in Plants. He married Helga Sorensen and reared three sons: Philip, John, and James. Some time after her passing, he married Virgie Eastburn Fry, who is the mother of two children: David and Diane. Stan and Virgie are the proud grandparents of Brandon and Brienne (from Philip), and Christopher, Julia, and Melissa (from Diane), and two great grandchildren, Gavin and David.

In 1984, Dr. Peloquin was elected to the U.S. National Academy of Sciences and was awarded a life membership by the Potato Association of America. In 1983, Dr. Peloquin was named the Campbell Bascom Professor by the University of Wisconsin and received Laurea Honoris Causa from the UniversitÓ degli Studi di Napoli "Federico II" (Italy) in 2002. These recognitions he has received for his innovative research and scientific leadership are an acknowledgment of his superb skills as a potato geneticist and breeder. Peloquin always points out that potato should be seen not only as an important food (fresh or processed) but also as the raw material for the starch-processing industry, as feed because its vines can be fed to animals, and as a potential resource for medicine because of the compounds in its true seed, and as an ornamental.


Peloquin's achievements are the result of fundamental, ingenious scientific insight, using potato genetics as the focal point for his research. Stan integrates his efforts with a broad range of networking activities, creating a world-famous "school" composed largely of his graduate students and research fellows. This intense collaboration has had a great impact in generating basic knowledge and for achieving practical methods for genetic enhancement.

Peloquin's early efforts were in broadening the basic knowledge about cytogenetics and genetics of potato. Stan's collaboration with Robert W. Hougas at the beginning of his career was an important first step, which led to more than 40 years of seminal potato research. Peloquin and Hougas, sometimes with early students and other collaborators, published 19 papers together that established a foundation of potato reproductive biology, genetics, and the genetics of reproductive biology that supported the broad and deep fundamental and applied developments that were to follow. Because of these early successes, Stan Peloquin set up germplasm enhancement methods relying on scaling up and scaling down chromosome sets via ploidy manipulations. Such ploidy manipulations are easily achieved in potato to transfer genes from wild Solanum species to the primary crop gene pool-particularly alleles for improving horticultural traits. His students in the classrooms of the University of Wisconsin still remember one of his favorite comments when teaching cytogenetics: "The best plant with which to manipulate individual chromosome numbers is wheat. The best plant with which to manipulate sets of chromosomes is potato." As Stan pointed out in many of his reviews: "The ability to obtain plants with the gamete chromosome number (haploids) and gametes with the plant chromosome number (2n gametes) is the basis for the ease of manipulating whole sets of chromosomes in potato."

Haploid and 2n Gamete Cytology and Cytogenetics

Scaling down the ploidy of the tetraploid potato (2n = 4x = 48 chromosomes) to the diploid level is achieved routinely by producing potato haploids (2n = 2x = 24 chromosomes). Maternal haploids can be easily obtained through parthenogenesis after interspecific hybridization of tetraploid cultivars with pollen of S. phureja Juz. et Buk. Haploid frequency is affected by both the maternal genotype and the pollen source, and both seed parent and pollen source influence the success of haploid production. Peloquin and co-workers indicated that the endosperm associated with a haploid embryo was always hexaploid, which clearly demonstrated the union of the two chromosome sets from S. phureja with the polar nuclei, and lack of fertilization of the egg. Hence, the pollen source influences haploid frequency via its effect on the endosperm. Paternal haploids are also obtained via anther culture, but maternal haploids offer more advantages for potato breeding because paternal haploid production requires gene(s) for androgenic competence, which are not always available in all tetraploid potato cultivars.

Gametes with the sporophytic chromosome number are referred to as 2n gametes. Some authors called them "numerically unreduced gametes," but Peloquin (and thereafter the students under his mentorship) avoided this term because normal gametes in any species have the haploid (n) number; i.e., 2n gametes would be 2x in diploids, 4x in tetraploids, and so on. Another reason he cautioned against the use of the term is that while chromosome number is not reduced in the formation of 2n gametes, the so-called "reduction division" typically does occur. The failure of students to appreciate this fact leaves them with an incomplete impression of the larger process. Premeiotic, meiotic, and postmeiotic abnormalities during gamete formation are correlated with the production of 2n gametes and there are at least six distinct possible modes of 2n gamete formation: premeiotic doubling, first division restitution (FDR), chromosome replication during meiotic interphase, second division restitution (SDR), postmeiotic doubling, and apospory. FDR and SDR mechanisms are respectively the most common modes of 2n pollen and 2n eggs formation in potato. In potato, FDR 2n gametes transmit on average 80% of the heterozygosity of the diploid progenitor to the tetraploid hybrid offspring. In contrast, SDR 2n gametes transfer on average less than 40% of the diploid heterozygosity to tetraploid hybrids.

Parallel orientation of the spindles in the second meiotic division is the most frequent mechanism of 2n pollen formation in most tuber-bearing Solanum spp. This meiotic abnormality is under the genetic control of the recessive gene ps (parallel spindles), which appears to be ubiquitous among Solanum species, but 2n pollen frequency could be affected by variable expressivity and incomplete penetrance. Omission of the second division after a normal first division appears to be the most common mode of SDR 2n egg formation in potato haploids, and haploid-species hybrids, which is controlled by a recessive meiotic mutant (os) in diploid potato. Genetic background and environment may affect the expressivity of this gene that also shows incomplete penetrance, and the frequency of modifier genes may enhance 2n egg expressivity.

Ploidy Manipulations for Genetic Enhancement

The findings of mechanisms that underlie parthenogenesis for producing maternal haploid plants and the formation of gametes with the parental chromosome number (or 2n gametes), established Peloquin's international prestige and credentials. Ploidy manipulations with haploids, 2n gametes, and wild species still remains today as one of the most impressive and exciting crop germplasm enhancement methods ensuing from cytogenetics research. In Stan's words "the potato is unsurpassed in the facility with which sets of chromosomes can be manipulated. This allows a germplasm enhancement strategy that involves species, haploids, 2n gametes and endosperm balance number (EBN). The species are the source of genetic diversity, haploids provide a method for 'capturing' the diversity, and 2n gametes and EBN are involved in an effective and efficient method of transmitting diversity to cultivars." There are two main methods for ploidy manipulations in potato: unilateral sexual polyploidization (4x-n gametes x 2x-2n gametes or vice versa) and bilateral sexual polyploidization (ensuing from crosses between 2x-2n gametes producing parents). For these breeding schemes, the diploid progenitors are developed by crossing potato haploids with tuber-bearing diploid species (2n = 2x = 24 chromosomes). Maternal haploids, which are easily extracted through parthenogenesis from most tetraploid cultivars, are crossed with diploid species for breeding at the diploid level. The locally adapted haploid-species hybrids are selected because they possess 2n gametes, acceptable tuber characteristics, and sometimes additional desired attributes, e.g., disease or pest resistance. Most of the hybrids ensuing from sexual polyploidization matings in potato are tetraploids. Triploid hybrids from tetraploid-diploid or diploid-diploid crosses are very rare due to a strong triploid block in potato.

The best diploid parents for tetraploid-diploid crosses are those producing FDR 2n gametes because hybrid vigor associated with high yields may be maximized by multi-allelism per locus in potato. Not surprisingly, tetraploid hybrids derived from diploid progenitors producing FDR 2n pollen often outyield their half-sib tetraploid hybrids from intermating tetraploid progenitors. However, maximum heterozygosity does not appear to be universal and depends on the genetic background of the crossing material, because of the importance of adaptation for the optimum expression of hybrid vigor in potato.

With this knowledge, Stan and his "school"-particularly in Italy, Poland, and the Centro Internacional de la Papa (CIP, Lima, Peru)-were able to develop new potato genotypes that combine high and stable yield, plus disease or pest resistance, which also allow the widening of potato growing in areas of the world that were previously unsuitable for this crop. The potato genotypes ensuing from their work were amenable to both fresh table markets and the chipping industry. Some of these materials became parents of cultivars now grown across the world. One potato cultivar ('Snowden') ensuing from conventional breeding work of Peloquin and colleagues at the University of Wisconsin is a leading chipping cultivar in North America.

Endosperm Balance Number (EBN) and Hybridization Barriers

The endosperm is a distinct trait among the Angiosperms, which results from double fertilization; one male gamete unites with the egg to form the zygote and the other male gamete with the central cell to form the endosperm, thereby making this tissue necessary for normal seed development. As Peloquin pointed out many times, "One of my colleagues defines endosperm as the tissue which along with potato feeds the world." Endosperm research by him and co-workers at the University of Wisconsin led to the Endosperm Balance Number (EBN) hypothesis to explain endosperm development in interploidy crosses, both intraspecific and interspecific. Under this theory, normal endosperm development only occurs when a balance of 2 EBN from the female parent matches with 1 EBN from the male parent in the resulting endosperm. Any deviations from this 2 maternal:1 paternal EBN ratio leads to faulty endosperm and lack of normal seed. For example, in 4x x 2x crosses and 2x x 4x crosses, endosperm development is regularly abnormal since the female:male ratios are 4:1 and 1:1, respectively. However, if 2n gametes function in the diploid, the ratio is 2:1 and endosperm development is normal. The fact that Mexican tetraploid species are easy to cross with most diploid species from South America-yielding triploid hybrids-suggested that both species sets possess 2 EBN, while they are unable to cross with Andean tetraploid species that are 4 EBN. This endosperm dosage system is typical of species possessing a "triploid block." However, triploids from crosses between tetraploids and diploids arise occasionally from misfertilization, mitotic abnormalities in the gametophyte, and/or mitotic misdivisions in the endosperm.

The above results led to the hypothesis-tested thereafter with success by many researchers worldwide, that EBN and 2n gametes are more important than actual chromosome number for predicting the success of crosses and the ploidy of the resulting progeny. The EBN of a species, which initially was thought to be controlled by a few genes rather than by the whole genome, determines, therefore, the effective ploidy, and natural gene flow may occur among species with distinct chromosome numbers but the same EBN. The EBN can be assigned for most Solanum species on the basis of the crossing behavior of a species with a standard species of known EBN.

Further research by some of his former co-workers demonstrated that three genes control EBN in potato, and gave convincing evidence for the participation of the EBN incompatibility system and 2n gametes in the origin and evolution of polyploids in tuber-bearing Solanum species. In this regard, the EBN determined gene pools among potato and wild Solanum species. The primary gene pool consists of old and modern tetraploid cultivars, tetraploid Andean landraces, and tetraploid breeding populations (i.e., 4 EBN polysomic polyploid tetraploid species). Diploid cultivars or breeding populations and diploid tuber-bearing Solanum species (2 EBN) producing 2n gametes and hexaploid (4 EBN) species also belong to this primary gene pool. The secondary gene pool includes disomic tetraploid (2 EBN) and diploid (1 EBN) tuber-bearing Solanum species, which may cross with the crop primary gene pool after isolation barriers (mainly due to EBN) are overcome. Wild diploid nontuber bearing Solanum species (1 EBN) of the series Etuberosa are in the tertiary potato gene pool, which could only cross with the primary crop gene pool through bridge species and embryo rescue. Other researchers expanded the EBN theory to many plant taxa, as a unifying concept to predict endosperm function in intraspecific-interploidy or interspecific crosses.


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