Wayne W. Hanna

Is supernumerary chromatin involved in gametophytic apomixis of polyploid plants

Abstract  Gametophytic apomixis, or unreduced embryo sac development that results in asexual reproduction through seeds, occurs in several families of angiosperms and must be polyphyletic in origin. The molecular mechanisms underlying gametophytic apomixis have not been discovered and are the subject of intense investigation. A common feature of almost all apomicts is their polyploid nature. From genetic mapping studies in both monocots and dicots, there is low genetic recombination associated with a single (rarely two), dominant locus for either aposporous or diplosporous embryo sac formation. In Pennisetum squamulatum and Cenchrus ciliaris, some DNA sequences mapping to the apospory locus are unique to apomictic genotypes and apparently hemizygous. This sequence divergence at the apomixis locus could be a consequence of genome rearrangements and isolation from genetic recombination, both of which may have contributed to the definition of a chromosomal region as supernumerary. The possible involvement of supernumerary chromatin, formed as a result of interspecific hybridization, in the origin of apomixis, is explored here.

Non-Mendelian transmission of an apospory-specific genomic region in a reciprocal cross between sexual pearl millet (Pennisetum glaucum) and an apomictic F1 (P. glaucum×P. squamulatum)

Abstract We recently showed that aposporous apomixis, a form of gametophytic apomixis, is controlled by a single apospory-specific genomic region (ASGR) in both Pennisetum squamulatum and Cenchrus ciliaris. We present evidence that in a reciprocal cross between sexual pearl millet (P. glaucum) and an apomictic F1 (P. glaucum× P. squamulatum) the ASGR is not transmitted at the same rate. When pearl millet was used as the female parent and the apomictic genotype as the pollen donor, the ASGR was transmitted at a rate of 0.41 in a progeny of 57 plants, indicating a slight transmission ratio distortion. However, in a population of 52 rare sexual progenies characterized among a large progeny of a quasi-obligate apomict (an F1 hybrid of P. glaucum×P. squamulatum), the transmission rate of ASGR was only 0.12. This strong segregation distortion may have occurred at four different levels: (1) female meiosis, (2) during female gametophyte maturation, (3) upon fertilization with differential survival of embryos being a consequence of differential gene expression controlled by parent-of-origin specific effects (imprinting) and (4) at a later developmental stage of the embryo through an embryo/endosperm genetic incompatibility system.

In vitro digestion and textural strength of rind and pith of normal and brown midrib stems

Abstract Intact sections of pith and rind from normal (N) and near-isogenic brown midrib (bmr) mutants of TxB623 sorghum ( Sorghum bicolor L. Moench.), GA 337 sudangrass ( Sorghum bicolor L. Moench.), and pearl millet ( Pennisetum glaucum (L.) R. Br.) were analyzed for loss of soluble components, in vitro dry matter digestion after incubation (24 h for pith and 72–120 h for rind) with rumen microorganisms, and textural strength using an Instron universal measuring instrument before and after in vitro digestion. Piths from all sudangrass and sorghum plants from the vegetative harvest were extensively degraded (about 900 g kg −1 ), with only vascular bundles remaining. In contrast to those of pearl millet, N and counterpart bmr piths from either sudangrass or sorghum were not different ( P > 0.05 ) in digestibility at 24 h. Pith digestibilities is sudangrass ranged from about 940 g kg −1 for vegetative stems to about 600 g kg −1 for mature, upper stems. Similarly, sorghum piths decreased from 840 g kg −1 in vegetative plants to about 690 g kg −1 in mature, upper stems. In all plants, bmr rinds were greater ( P ≤ 0.05) or tended to be greater ( P ≤ 0.10) in digestibility than N rinds. For example, sudangrass digestibilities for N and bmr rinds, respectively, were 736 g kg −1 and 901 g kg −1 for vegetative plants and 606 g kg −1 and 808 g kg −1 for mature, upper plants. Shear stress (newtons per area) of piths were about 10-fold less than that of rinds when all plants were considered. Differences were small and inconsistent for undegraded piths and rinds within N/bmr comparisons for sudangrass and sorghum. Extended disruption by rumen microorganisms of pith and rind sections of sudangrass and sorghum prevented testing for shear stress of most digested samples. In pearl millet, rinds in either bmr mutant were about 40% weaker than those in N plants. In study, shear stresses of pith and rind were not consistently different within N/bmr comparisons, and overall results on textural strength did not relate well to variations observed in digestibility for N and counterpart bmr stems.

Genetic variation for Mg tissue concentration in pearl millet lines grown under Mg stress conditions.

Abstract Efficient nutrient utilization by plants is extremely important in developing countries and is becoming more important in the developed countries as the costs of fertilizers are increasing. Pearl millet is a world food and forage crop usually grown on droughty soils low in nutrients. Eighty‐five genetically diverse pearl millet, Pennisetum glaucum (L.) R. Br. lines were grown in low Mg and K sand for three weeks to screen for efficient Mg uptake. Magnesium free Hoaglands solution at one‐fourth strength was used to water the plants as needed. Forage produced was clipped, dried, ground, ashed, and analyzed for Mg and K content. A 2‐fold difference for Mg concentration in the forage existed among the lines. The data indicated that a low [K]/[Mg] ratio in the plant was indicative of a Mg efficient genotype. The data also suggest that caution should be used in making general nutrient recommendations based on tissue or soil tests without knowing the nutrient requirement of the cultivar.