Vernon Gracen

Conventional breeding, marker-assisted selection, genomic selection and inbreeding in clonally propagated crops: a case study for cassava

Key messageConsolidates relevant molecular and phenotypic information on cassava to demonstrate relevance of heterosis, and alternatives to exploit it by integrating different tools. Ideas are useful to other asexually reproduced crops.AbstractAsexually propagated crops offer the advantage that all genetic effects can be exploited in farmers’ production fields. However, non-additive effects complicate selection because, while influencing the performance of the materials under evaluation, they cannot be transmitted efficiently to the following cycle of selection. Cassava can be used as a model crop for asexually propagated crops because of its diploid nature and the absence of (known) incompatibility effects. New technologies such as genomic selection (GS), use of inbred progenitors based on doubled haploids and induction of flowering can be employed for accelerating genetic gains in cassava. Available information suggests that heterosis, non-additive genetic effects and within-family variation are relatively large for complex traits such as fresh root yield, moderate for dry matter or starch content in the roots, and low for defensive traits (pest and disease resistance) and plant architecture. The present article considers the potential impact of different technologies for maximizing gains for key traits in cassava, and highlights the advantages of integrating them. Exploiting heterosis would be optimized through the implementation of reciprocal recurrent selection. The advantages of using inbred progenitors would allow shifting the current cassava phenotypic recurrent selection method into line improvement, which in turn would allow designing outstanding hybrids rather than finding them by trial and error.

Ultrastructural effects of Helminthosporium maydis race T toxin on mitochondria of corn roots and protoplasts

Zea mays inbred W64A in Texas (T, toxin sensitive) male sterile and non-male sterile (N, toxin resistant) cytoplasms were utilized. Roots of freshly germinated seeds were treated for 15 min of 2 hr with culture filtrate from liquid grown Helminthosporium maydis Race T, or with a chloroform extractable purified fraction from the culture filtrate. In the susceptible W64A T line, toxin treatment, both crude and purified, caused swelling and loss of matrix densiy in mitochondria of root cap and vacuolated cells in the region of elongation. One hour treatment with the chloroform extractable toxin fraction caused similar effects or mitochondria of isolated leaf protoplasts. This is the first report of such rapid in vivo effects of HmT toxin on mitochondria. Difficulty in obtaining consistent preservation of meristem mitochondria precluded drawing firm conclusions concerning that region of the root. In the resistant W64A N line, protoplast and root mitochondria were unaffected by the toxin.

Fertile revertants from S-type male-sterile maize grown in vitro

SummaryPlants were regenerated from callus cultures of maize inbred W182BN with the S(USDA) type of cytoplasmic male sterility (cms). Some regenerates from 16 of 18 separate cultures had fertile tassels. Many other regenerates, whose fertility could not be scored accurately because of abnormal plant morphology, produced fertile progeny after pollination with N cytoplasm W182BN. Revertant plants and/or progeny were obtained from all 18 cultures, which included the CA, D, LBN, and S sources of cmsS. More revertants were recovered from cultures maintained as callus for 12 months than from 3–4 month old cultures. Several types of evidence (absence of segregation for fertility after selfing or pollination of revertants with standard W182BN, pollen viability counts, failure of revertants to restore sterile cmsS lines to fertility, mitochondrial DNA analyses) indicated that the reversion to fertility involved cytoplasmic rather than nuclear alterations. All revertants examined lacked the S1 and S2 plasmid-like DNAs characteristic of the mitochondrial genome of sterile cmsS lines. Most callus cultures lost S1 and S2 after 13–20 months in vitro. No revertants were seen among thousands of W182BN cmsS plants grown from seed in the field or among plants from tissue cultures of W182BN with the C or T types of cms. The cytoplasmic revertants recovered from culture may be useful for the molecular analysis of cmsS.

Bioassays for detection and quantification of Helminthosporium maydis race T-toxin: a comparison

Abstract Sensitivities of seven commonly used bioassays for Helminthosporium maydis race T-toxin were determined. The most easily quantified and highly sensitive bioassays were based either on dark CO2 fixation by green leaves or on respiration rates of isolated mitochondria. These bioassays were 5 to 10 times more sensitive than was the semiquantitative seedling root growth bioassay. Bioassays based on the production of visible lesions on leaves did not provide reliable dosage-response relationships and were estimated to be 20 to 100 times less sensitive to toxin than dark CO2 fixation or mitochondrial respiration. Choice of bioassay depends on the objective of the experiment in which it is used.

Somaclonal Variation in Progeny of Plants from Corn Tissue Cultures

Spontaneous variation in plants recovered from tissue culture has been documented for many species (1). This variation, recently termed “somaclonal variation” (1), has been seen in many types of culture systems, including ones involving protoplasts, long-term callus cultures, and fresh explants. The occurrence of somaclonal variation has both positive and negative aspects. For those concerned with in vitro propagation, it is undesirable; progeny not true-to-type are usually of little value. Spontaneous changes may also be a problem in attempts to transform plant cells; a high frequency of change not related to the experimental manipulations can complicate interpretation of results and can yield material with alterations other than the desired specific gene transfer. Although variability among cultured plant cells may reduce or obviate the need for mutagenesis prior to in vitro selection, it can again result in variants other than those being specifically selected.

Fertility restoration and mitochondrial nucleic acids distinguish at least five subgroups among cms-S cytoplasms of maize (Zea mays L.)

SummaryDifferences in fertility restoration and mitochondrial nucleic acids permitted division of 25 accessions of S-type male sterile cytoplasm (cms-S) of maize into five subgroups: B/D, CA, LBN, ME, and S(USDA). S cytoplasm itself (USDA cytoplasm) was surprisingly not representative of cms-S, since only two other accessions, TC and I, matched its mitochondrial DNA pattern. CA was the predominant subgroup, containing 18 of the 25 accessions. The B/D and ME subgroups were the most fertile and LBN the most sterile. The exceptional sterility of LBN cytoplasm makes it the most promising of the 25 cms-S accessions for the production of hybrid seed. The most efficient means of quantifying the fertility of the subgroups was analysis of pollen morphology in plants having cms-S cytoplasm and simultaneously being heterozygous for nuclear restorer-of-fertility (Rf) genes. This method took advantage of the gametophytic nature of cms-S restoration. The inbred NY821LERf was found to contain at least two restorer genes for cms-S. Fertility differences were correlated with mitochondrial nucleic acid variation in the LBN, ME, and S (USDA) subgroups.

LBN, a male-sterile cytoplasm of maize, contains two double-stranded RNAs

Abstract Two nucleic acids species discovered in a male-sterile cytoplasm of maize have been shown to be double-stranded RNAs (dsRNAs) of apparent mol. wt. 1.9 and 0.5 × 10 6 . This cytoplasm has been designated ‘LBN’ for ‘L cytoplasm in inbred W182BN’. The dsRNAs are associated with mitochondria and are maternally transmitted. Nuclear genotype influences the amount of dsRNA recoverable.

Diversity analysis of sweet potato ( Ipomoea batatas [L.] Lam) germplasm from Burkina Faso using morphological and simple sequence repeats markers

Collecting and characterizing plant material has been basic for crop improvement, and diversity has long been seen as vital for rational management and use of crops. Thirty (30) morphological characters and thirty (30) simple sequence repeat (SSR) markers were used to assess the diversity among 112 sweet potato (Ipomoea batatas [L.] Lam) cultivars in Burkina Faso and to develop a core collection. Eight morphological characters were able to differentiate the 112 accessions and to identify 11 duplicates while 28 SSR markers were more informative in discriminating the accessions and to identify five duplicates. The diversity assessment using the two approaches revealed high diversity with a coefficient of 0.73 using the phenotypic data, while moderate diversity with a coefficient of 0.49 was obtained using the SSR markers. These results show no correlation between the two approaches (with dissimilarity index of 0.95). A core collection was constituted using the SSR based data while the eight discriminative phenotypic descriptors will be used in the identification of cultivars.

Mitochondrial DNA duplication/deletion events and polymorphism of the C group of male sterile maize cytoplasms

SummaryFive accessions of members of the C group of male sterile maize cytoplasms (BB, C, ES, PR, and RB) in two nuclear backgrounds (A619 and A632) were examined to elucidate the nature of mitochondrial genome diversity within a related group of cytoplasms. Cosmid and plasmid clones carrying single copy and recombinationally active sequences from N and S cytoplasms of maize were used as probes. Although restriction patterns are quite similar, each of the five could be discriminated by evidence of sequence duplication and recombination, deletion of recombinationally active sequences of N, normal cytoplasm, population of mini-circular DNAs, and by restriction patterns. Each member of the group carried a 1,913 bp minicircular mtDNA, while all entries but RB carried a 1,445 bp minicircular mtDNA. Members of the C group clearly are not molecularly identical; evolution of the group included principal genome reorganization involving sequence duplication/deletion events, apparently independent of the cms trait.

Southern corn leaf blight disease: studies on mitochondrial biochemistry and ultrastructure

The study of southern corn leaf blight disease is of considerable economic and scientific importance. The economic consequences of the disease were dramatically illustrated in 1970 when an outbreak of almost epidemic proportions destroyed a large part of the U.S. corn crop. Scientifically, southern corn leaf blight disease represents a fascinating system for the study of fungal toxin action and, in addition, provides us with a unique tool for the study of higher plant mitochondrial genetics. Southern corn leaf blight disease is caused by the fungus Helminthosporium maydis Race T. This organism is representative of fungal pathogens that produce host specific toxic chemicals which adversely affect susceptible host plants and result in loss of yield (32, 35). Susceptibility to H. maydis Race T toxin (HINT toxin) is controlled primarily by cytoplasmic genes. Texas (T) male-sterile cytoplasm is susceptible but the other male-sterile types (C, S) and the non male-sterile (N) are resistant (19, 31). Certain nuclear genes can restore the male-fertility of T cytoplasm corn (6). Other nuclear genes affect the resistance of T cytoplasm plants to H. maydis Race T (18, 24), but it is unclear whether nuclear genes affect toxin response directly (27). In any event, the effects of nuclear genes are secondary to cytoplasmic genes in controlling susceptibility to H. maydis Race T and HmT toxin. There is a great deal of interest in the nature of HInT toxin sensitivity. The many effects of HInT toxin on T