Academic journal article Genetics

Effect of Teosinte Cytoplasmic Genomes on Maize Phenotype

Academic journal article Genetics

Effect of Teosinte Cytoplasmic Genomes on Maize Phenotype

Article excerpt

ABSTRACT

Determining the contribution of organelle genes to plant phenotype is hampered by several factors, including the paucity of variation in the plastid and mitochondrial genomes. To circumvent this problem, evolutionary divergence between maize (Zea mays ssp. mays) and the teosintes, its closest relatives, was utilized as a source of cytoplasmic genetic variation. Maize lines in which the maize organelle genomes were replaced through serial backcrossing by those representing the entire genus, yielding alloplasmic sublines, or cytolines were created. To avoid the confounding effects of segregating nuclear alleles, an inbred maize line was utilized. Cytolines with Z. mays teosinte cytoplasms were generally indistinguishable from maize. However, cytolines with cytoplasm from the more distantly related Z. luxurians, Z. diploperennis, or Z. perennis exhibited a plethora of differences in growth, development, morphology, and function. Significant differences were observed for 56 of the 58 characters studied. Each cytoline was significantly different from the inbred line for most characters. For a given character, variation was often greater among cytolines having cytoplasms from the same species than among those from different species. The characters differed largely independently of each other. These results suggest that the cytoplasm contributes significantly to a large proportion of plant traits and that many of the organelle genes are phenotypically important.

THE phenotype of a eukaryote is determined primarily, but not entirely, by its nuclear genome. In plants, literally thousands of nuclearly controlled phenotypes have been described in a wide variety of agronomic and nonagronomic species. The genes underlying these phenotypes are almost always inherited in a Mendelian fashion, so that it is possible to discover, investigate, and manipulate them in a fairly easy and straightforward manner. Genomes of plastids and mitochondria are also known to play a role in the growth, development, and well-being of a plant, but this role is generally presumed to be small or invariant. Despite the genetic simplicity of organelles, the actual extent of this role and what effect cytoplasmic genes have on plant phenotype is largely undetermined. The cytoplasmically associated variation that has been observed in plants is in fact limited, in part because organelle DNA sequence is very highly conserved within plant species and often within genera (WoLFE et al. 1987). Since plant organelles are in most cases strictly uniparentally inherited (SOLIMAN et al. 1987; REBOUD and ZEYL 1994), and thus not subject to Mendelian assortment, cytoplasmic effects are usually notable only in comparisons of reciprocal crosses. Furthermore, because multiple organelle genomes are inherited within each organelle, the effect that any new mutation might engender is typically not exposed because it is masked by dozens to thousands of nonmutant genomes.

Most described cytoplasmically inherited phenotypes are limited to the obvious, such as lack of greeninggenerally due to nuclear-plastid incompatibility (KiRK and TILNEY-BASSETT 1978)-or cytoplasmic male sterility (CMS)-exclusively associated with nuclear-mitochondrial incompatibility (LEAVER et al. 1988; HANSON 1991). Plants that suffer lack of greening are, not unexpectedly, often subject to growth and development deficiencies, whereas plants that exhibit CMS, while failing to produce or shed functional pollen, are usually otherwise lacking in gross phenotypic abnormalities (e.g., LAUGHNAN and GABAY-LAUGHNAN 1983).

Additional cytoplasmically inherited effects, such as tissue-culture regeneration ability (e.g., EKIZ and KONZAK 1991), pathogen resistance (e.g., VOLUEVICH and BULOICHIK 1992), seed starch type (e.g., POONI et al. 1993b), yield (e.g., LOESSL et al. 2000), tolerance of cold (e.g., HUTTON and LOY 1992) and heat (e.g., SHONNARD and GEPTS 1994), and high-salt (Hou et al. …

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