Half tetrad analysis in alfalfa using multiple restriction fragment length polymorphism markers.

A maximum likelihood approach of half tetrad analysis (HTA) based on multiple restriction fragment length polymorphism (RFLP) markers was developed. This procedure estimates the relative frequencies of 2n gametes produced by mechanisms genetically equivalent to first division restitution (FDR) or second division restitution and simultaneously locates the centromere within a linkage group of RFLP marker loci. The method was applied to the diploid alfalfa clone PG-F9 (2n = 2x = 16) previously selected because of its high frequency of 2n egg production. HTA was based on four RFLP loci for which PG-F9 was heterozygous with codominant alleles that were absent in the tetraploid tester. Models including three linked and one unlinked RFLP loci were developed and tested. Results of the HTA showed that PG-F9 produced 6% FDR and 94% second division restitution 2n eggs. Information from a marker locus belonging to one linkage group was used to more precisely locate the centromere on a different linkage group. HTA, together with previous cytological analysis, indicated that in PG-F9, FDR 2n eggs are likely produced by diplospory, a mechanism common among apomictic species. The occurrence of FDR 2n eggs in plant species and their importance for crop evolution and breeding is discussed together with the potential applicability of multilocus HTA in the study of reproductive mutants.

An overview of a feasibility study for the production of industrial enzymes in transgenic alfalfa.

The overall aims of the research are to develop genetically engineered alfalfa producing high levels of industrially important enzymes and to develop rapid methods for extracting and purifying these enzymes from alfalfa juice. Using a reporter gene beta-glucuronidase (GUS) as a model system, we were able to demonstrate production of a foreign protein in alfalfa and gain valuable insight into the molecular approaches required for the expression and accumulation of foreign proteins in leaf tissue. GUS activity varied among individual transformants, and GUS was expressed in all plant tissues. GUS activity was shown to segregate in sexual progeny. There was no correlation between copy number of the GUS gene and activity. We have recently demonstrated the production of Mn-dependent lignin peroxidase and alpha-amylase in transgenic alfalfa. Concurrent research in the agricultural engineering aspects of this feasibility study focused on extraction strategies for the recovery of alfalfa juice, and on an evaluation of methods for processing and concentrating the juice. Thus, we are in a position to use plants expressing enzymes that have current or potential industrial importance to complete a feasibility study, and determine whether we can indeed economically recover target enzymes from field-grown transgenic alfalfa plants. The technology developed for these enzymes can be used to extract other value-added products from plants in the future.

Relationships among genetic distance, forage yield and heterozygosity in isogenic diploid and tetraploid alfalfa populations.

Isogenic diploid and tetraploid alfalfa (Medicago sativa L.) was studied with molecular markers to help understand why diploid performance and breeding behavior does not always predict that of tetraploids. In a previous study of partially heterozygous alfalfa genotypes, we detected a low correlation between yields of isogenic diploid (2x) and tetraploid (4x) single-cross progenies, and genetic distances were more highly correlated with yields of tetraploids than diploids. These differences may be related to the level of RFLP heterozygosity expected among progenies derived from heterozygous parents at the two ploidy levels. The objectives of this study were to determine the relationships among genetic distance, forage yield and heterozygosity in isogenic 2 x and 4 x alfalfa populations. Four diploid genotypes were chromosome doubled to produce corresponding isogenic autotetraploids, and these genotypes were mated in 4 × 4 diallels to produce 6 single-cross families at each ploidy level for field evaluation. Allele compositions of parents were determined at 33 RFLP loci by monitoring segregation of homologous restriction fragments among individuals within progenies, and these were used to estimate RFLP heterozygosity levels for all single-cross progenies at both ploidy levels. RFLP heterozygosity rankings were identical between progenies of isogenic diploid and tetraploid parents; but significant associations (P < 0.05) between estimated heterozygosity levels and forage yield were detected only at the tetraploid level. Since tetraploid families were nearly 25% more heterozygous than the corresponding diploid families, inconsistencies in the association between molecular marker diversity and forage yields of isogenic 2 x and 4 x single crosses may be due to recessive alleles that are expressed in diploids but masked in tetraploids. The gene action involved in heterosis may be the same at both ploidy levels; however, tetraploids benefit from greater complementary gene interactions than are possible for equivalent diploids.

Unreduced gametes in diploid Medicago and their importance in alfalfa breeding.

In the genus Medicago, it is known that 2n gametes have been important in the evolution and breeding of cultivated alfalfa, which is a natural polysomic polyploid (2n=4x=32), however little is known on the frequency of male and female 2n gametes in diploid relatives of alfalfa. To obtain data on the frequency of 2n gametes, more than 12,000 2x-4x and 4x-2x crosses were made in 1982 at Madison (USA). Diploid parents in crosses were from four populations of M. coerulea, two of M. falcata and one diploid population of cultivated M. sativa which was derived by haploidy. The tetraploid seed parent in the crosses was a male-sterile M. sativa clone and vigorous tetraploid M. sativa plants were used as pollen parents. Each of 274 diploid plants was utilized both as male and as female. Of the 548 cross combinations, 266 crosses produced variable quantities of seeds which were sown in 1983 in a greenhouse at Perugia (Italy); the plants were subsequently space transplanted in the field in 1984. The identification of ploidy level of these genotypes was made on the basis of morphological characters, plant fertility, pollen stainability and chromosome counts.Of the 515 plants analyzed, the majority behaved as normal tetraploids indicating that many diploid plants produced 2n gametes. Diplogynous and diplandrous gamete production was not correlated with each other, which indicated a different genetic control of 2n sporogenesis in the 2 sexes. Only 4 F1 triploid plants confirmed the presence of a very effective triploid block in alfalfa. In consequence, bilateral sexual polyploidization is a more likely alternative for the origin of tetraploid alfalfa than triploid bridges. The present study showed that it is possible to efficiently identify genotypes able to produce high frequencies of 2n gametes within natural populations of diploids Medicago that are useful in alfalfa breeding.

An unstable anthocyanin mutation recovered from tissue culture of alfalfa (Medicago sativa) : 1. High frequency of reversion upon reculture.

A white-flowered mutant ("WFM") was regenerated from tissue culture of a purple-flowered plant of tetraploid alfalfa (Medicago sativa L.). When WFM was recultured, many regenerated plants (>20%) were purple-flowered. Genetic analysis established that a functional allele, C2, of a locus required for anthocyanin pigmentation was in the simplex condition (C2c2c2c2) in the donor genotype when it mutated to an unstable recessive ("mutable") allele, c2-m4, which is carried by WFM. Tissue culture experiments demonstrated that c2-m4 reverts to function at a high frequency in vitro. Results indicate that reversion occurs early in culture and may be the result of a genome shock associated with callus formation. Reversion also occurs in planta, but at a much lower frequency than in vitro. The c2-m4 allele is transmitted to progeny which revert in tissue culture. Revertant alleles, like the progenitor allele, are stable and are sexually transmitted. The action of a transposable element which is especially active in vitro is suggested.

An unstable anthocyanin mutation recovered from tissue culture of alfalfa (Medicago sativa) : 2. Stable nonrevertants derived from reculture.

An unstable recessive ("mutable") allele, c2-m4, of a locus required for anthocyanin pigmentation in alfalfa (Medicago sativa L.) reverts to a stable functional state at high frequency in vitro. It was previously established that a white-flowered mutant ("WFM") and a white-flowered progeny of WFM ("WHGW3") each carry the unstable allele. More than 20% of plants regenerated from tissue cultures of WFM and WHGW3 are revertant. It is here established that most nonrevertant plants regenerated from cultures of WFM and WHGW3 are stabilized in the recessive condition. Reculture of nonrevertants of WFM and WHGW3 indicated that there are three classes of nonrevertants: (i) Nonrevertants which revert in vitro at a high frequency typical of WFM; (ii) Nonrevertants which revert upon reculture but at significantly lower frequencies than WFM; and (iii) Nonrevertants which do not revert upon reculture. These observations are discussed in terms of transposable element action in vitro.

Comparisons of alfalfa somaclonal and sexual derivatives from the same genetic source.

Tetraploid alfalfa (Medicago sativa L.) clones were derived from the same diploid genetic background by four different methods. A phenotypically superior clone was selected from each method and compared for herbage yield and fertility. The four methods and their best clones were: a) In vitro somatic chromosome doubling of one diploid hybrid (HG2-4x); b) selection within a two allele tetraploid synthetic population derived from HG2-4x (HAG); c) somaclonal variant selection from cell suspension culture of the diploid hybrid (NS1); and d) sexual polyploidization of a sibling hybrid (HXG). Clones HG2-4x, HAG, and NS1 were likely diallelic or monoallelic at all loci. Clone HXG was probably tetrallelic or triallelic at most loci. Experiments measured fertility, clonal herbage yield, and herbage yield of test cross progeny for each selected clone. Fertility rankings were HXG = HAG > NS1 > HG2-4x. Clonal herbage yield rankings were HXG = HAG > NS1 > HG2-4x. Test cross progeny herbage yield rankings varied depending on the tester, but, in general, HXG ≧ HAG ≧ NS1 HG2-4x. Overall the best clones from the sexual methods exceeded the best somaclonal variant which, in turn, was better than the chromosome doubled clone.

Selection and characterization of ethionine-resistant alfalfa (Medicago sativa L.) cell lines.

Diploid alfalfa (HG2), capable of plant regeneration from tissue culture, was used to select variant cell lines resistant to growth inhibition due to ethionine (an analog of methionine). Approximately 10(7) suspension-cultured cells were mutagenized with methane sulfonic acid ethylester and then plated in solid media containing ethionine. Callus colonies formed on media with 0.02 mM ethionine. Of the 124 cell lines recovered, 91 regenerated plants. After six months growth on media without ethionine, 15 of 110 cell lines of callus grew significantly better than HG2 on 1 mM ethionine. Several ethionine-resistant callus cultures were also resistant to growth inhibition due to the addition of lysine + threonine to the media. High concentrations, relative to unselected HG2 callus, of methionine, cysteine, cystathionine, and glutathione were found in some, but not all, ethionine-resistant callus cultures. Cell line R32, which had a ca. tenfold increase in soluble methionine, had a 43% increase in total free amino acids and a 40% increase in amino acids in protein as compared to unselected HG2 callus. Relative amounts of each amino acid in protein were the same in both.