Gurdev S. Khush

Cytogenetic analysis of the tomato genome by means of induced deficiencies

Cytological studies of 74 deficiencies of tomato chromosomes induced by radiation and identified by the pseudo-dominant technique reveal the loci of 35 genes on 18 of the 24 arms of the complement. These findings integrated with data obtained from various trisomic types establish centromere positions, orientation of linkage groups, and markers on all but three of the arms. The prospects of obtaining a specific kind of deficiency for a given region were found to depend on : (1) kind of radiation applied, (2) (non-random) breakage frequency in different parts of the chromosome, (3) stability of broken ends, (4) tolerance of deficiency in different parts of the genome, and (5) relative vigor of the mutant homozygote used to detect the deficiency. Aspects of the frequently observed non-homologous pairing phenomenon are presented and discussed. Marker genes whose loci are known appear to be non-randomly distributed between and within chromosomes. Chromosome exchanges as determined by genetic crossing over and cytologically observed chiasmata are likewise non-randomly distributed between and within chromosomes.

Genetic Activity in a Heterochromatic Chromosome Segment of the Tomato

The first example of genetic activity within a heterochromatic region of the tomato is provided by the delimitation of nv to the long arm of chromosome 9 by means of the induced deficiency method. A close spatial relationship between nv and ah was established by deficiencies for ah in the same arm and by linkage tests between the two genes.

Meiosis in hybrids betweenLycopersicon esculentum andSolanum pennellii

Meiotic chromosome cytology was compared betweenSolanum pennellii, Lycopersicon esculentum, and the F1 hybrid. Pachytene chromosomes are very similar in gross morphology, but several of theS. pennellii chromosomes were found to have somewhat longer chromatic regions with discrete chromomeres, and darkly staining chromomeres in the achromatic regions.Little evidence could be found for the existence of rearrangements between chromosomes of the two species. With respect to chromomere pattern, on the other hand, a number of differences were seen. Meiosis in the hybrid is strictly regular. Only size inequalities occur in certain bivalents.Considering the evidence from chromosome pairing, hybridization compatibility, hybrid fertility, and plant morphology, it is concluded that the phylogenetic relationship is much closer betweenS. pennellii andL. esculentum than it is between either one andS. lycopersicoides. Attention is called to the present unsatisfactory placement ofS. pennellii and to the need for revising the taxonomy to place it andL. esculentum in the same genus, possibly in the same subgeneric category.

Cytogenetic and evolutionary studies inSecale III. Cytogenetics of weedy ryes and origin of cultivated rye

SummarySeveral weedy forms of rye which have been variously recognized as varieties, subspecies or even species, and a new collection of primitive rye were crossed with the cultivated variety ‘Merced.’ They all crossed readily and produced vigorous F1’s with normal meiosis and with F1 pollen fertility nearly as high as that of the parental materials. No evidence of structurel differences between the genomes of weedy ryes and cultivated rye was found. On the grounds of similar chromosome arrangements, identical breeding habits, equal periodicity, morphological continuity, crossability, regular meiotic behavior of their hybrids, normal F1 pollen fertility and geographical continuity, it is concluded that all these forms should be recognized as subspecies ofS. cereale.Study of the meristematic activity of the seedlings of four strains of rye showed that forms with larger grains had greater meristematic activity, while those with smaller grains had less. It is concluded that large-grained forms originated probably by the accumulation of mutations causing increase in grain size because large-grained forms had adaptive advantage due to their better competitive ability. Similarly, it is suggested that the stiff rachis evolved due to unconscious selection by primitive farmers for polygenes conditioning that character.A figure showing the distribution of weedy ryes and the centers of maximum diversity is given. It is concluded that rye was first domesticated at several places independently and at different times. Cultivation of rye probably entered Europe by two routes: through northern Caucus and through central Asia.

Studies on the linkage map of chromosome 4 of the tomato and on the transmission of induced deficiencies

Various genetic and cytogenetic techniques were applied to an analysis of the linkage map of chromosome 4-a chromosome that is considered to be representative of the tomato complement. Loci have been approximated by standard F2 linkage tests for 18 genes, including six on the short arm and 12 on the long arm, covering a map distance of 132 units (c.m.). The loci of four key markers were approximated on pachytene chromosomes by a study of radiation-induced deficiencies:clau near the end of the short arm,ful near the euchromatic-heterochromatic boundary of the short arm,ra near the same region on the long arm, ande in the middle of the long arm. Normal transmission for a presumedra deficiency suggests that this gene lies in the heterochromatin of 4L. According to tertiary trisomic segregation,w-4, known by linkage test to be proximal tora, resides on 4L, therefore probably also in the heterochromatic region. The centromere is consequently delimited to a region of 4 c.m. betweenful andw-4. The resultant maps reveal a very much lower crossover rate within heterochromatin—estimated at 0.8 c.m./μ—than for euchromatin—estimated at 4.8 c.m./μ for the short arm and 5.7 for the long arm. Also apparent is a strong tendency of the genes to concentrate toward the centromere of the genetic map and in the proximal sections of the euchromatin of the cytological map.Studies were made of the genetic transmission of various small deficiencies on chromosome 4 as well as a newly discovered deficiency fornv on chromosome 9, supporting the following conclusions. Regardless of their size, deficiendies of euchromatin are not transmitted. Deficiencies of heterochromatin are transmitted to a varying extent depending on their size. A presumed deficiency forra that is too small to be detected cytologically was transmitted without adverse effect on gametes. Somewhat larger deficiencies may be transmitted at reduced rates by female gametes and the largest at extremely low rates, even on the female side.

The origin, identification, and cytogenetic behavior of tomato monosomics

The missing chromosomal elements were cytologically identified in a primary monosomic (haplo-11) and 18 tertiary monosomics (lacking interchanged chromosomes) induced by radiation in the tomato. For the tertiary monosomics all interchanges occurred in the centromeres, and, as with single arms deficiencies in the same materials, deficiencies are tolerated for only 15 of the 24 arms of the complement. Non-homologous pairing was frequently observed in the univalent pachytene chromosomes. The monosomic condition was not transmitted to any of the 11,981 progeny of ten tested monosomics. Reproductive fertility and gross morphology were also studied.

Genetic erosion over time of rice landrace agrobiodiversity

Changes in global biodiversity at the genetic level have proved difficult to determine for most organisms because of lack of standardized, repeated or historical data; this hampers the attempts to meet the convention on biological diversity (CBD) 2010 targets of reducing loss of genetic diversity, particularly of crop species. For rice, where germplasm and genetic data have been collected throughout South and Southeast Asia over many decades, contrary to popular opinion, we have been unable to detect a significant reduction of available genetic diversity in our study material. This absence of a decline may be viewed positively; over the 33-year timescale of our study, genetic diversity amongst landraces grown in traditional agricultural systems was still sufficiently abundant to be collected for ex situ conservation. However, if significant genetic erosion does take place in the future as a result of accelerating global warming and/or major changes in land use or agricultural practices, will it be catastrophic or gradual, and how will it be detected? We have shown a strong link between numbers of landraces collected (and therefore extant) and genetic diversity; hence, we have a clear indicator to detect loss of genetic diversity in the future. Our findings lend considerable support for ex situ conservation of germplasm; the more than substantial genetic resources already in genebanks are now safe. On the other hand, it is the germplasm growing in farmers fields, continually adapting genetically to changing environmental conditions and evolving novel genetic forms, whose future has been much less certain but can now be effectively monitored using our criteria.

Punjab’s Water Woes and India’s Food Security

Ground water tables in Punjab are declining at alarming rates in most districts of Punjab. One of the major causes of declining water tables is the increased cropping intensity. Whereas cropping intensity in Punjab was only 120% until about 50 years ago, it is now 190%. With one crop per year, a balance was maintained between water extraction and aquifer recharge. With two crops per year, this balance has been altered. Homogenization of crops in the state has also exacerbated the problem. Even more serious threat to nation’s agriculture is climate change. Himalayan glaciers, which are water towers for our rivers, are retreating. This will reduce the water flow in our rivers. While the climate-change impact on our water availability is several years away, we must address immediate problem of declining water tables in the state. Suggested interventions include crop diversification, precision agriculture, including water saving technologies, and developing crop varieties with improved water-use efficiency.

IR64: a high-quality and high-yielding mega variety

High-yielding varieties developed in the 1960s and 1970s at the International Rice Research Institute (IRRI) and elsewhere benefited farmers and the public, ultimately increasing yields and reducing the cost of rice to consumers. Most of these varieties, however, did not have the optimum cooking quality that was possessed by many of the traditional varieties they replaced. In 1985, the IRRI-developed indica variety IR64 was released in the Philippines. In addition to its high yield, early maturity and disease resistance, it had excellent cooking quality, matching that of the best varieties available. These merits resulted in its rapid spread and cultivation on over 10 million ha in the two decades after it was released. It has intermediate amylose content and gelatinization temperature, and good taste. It is resistant to blast and bacterial blight diseases, and to brown planthopper. Because of its success as a variety, it has been used extensively in scientific studies and has been well-characterized genetically. Many valuable genes have been introduced into IR64 through backcross breeding and it has been used in thousands of crosses. Its area of cultivation has declined in the past 10xa0years, but it has been replaced by a new generation of high-quality varieties that are mostly its progeny or relatives. Continued basic studies on IR64 and related varieties should help in unraveling the complex genetic control of yield and other desirable traits that are prized by rice farmers and consumers.

Norman Borlaug 1914 – 2009

An appreciation of the life of Norman Borlaug