Organelles and the Molecular Biology of Eukaryotes Help (page 3)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Inheritance of Organelles

In most plants and animals, mitochondria and chloroplasts are strictly inherited only from the female parent (maternal transmission) because the male gamete (or that part that enters into fertilization) is essentially devoid of these organelles. It is estimated that in about two-thirds of plant species the inheritance of chloroplasts is strictly maternal. Traits with an extranuclear basis may be identified on the basis of several diagnostic criteria.

  1. Differences in reciprocal crosses that cannot be attributed to sex linkage or some other chromosomal basis tend to implicate cytoplasmic factors.
    1. If progeny show only the characteristics of the female parent that can be attributed to unequal cytoplasmic contributions of male and female parents, then plasmagene inheritance is suspect (Example 13.24).
    2. If the uniparental inheritance of a trait cannot be attributed to unequal cytoplasmic contributions from the parents, this does not necessarily rule out cytoplasmic factors (Example 13.25).
  2. Extranuclear factors may be detected by either the absence of segregation at meiosis (Example 13.26) or by segregation that fails to follow Mendelian laws (Example 13.27).
  3. Repeated backcrossing of progeny to one of the parental types for several generations causes their chromosomal endowment to rapidly approach 100% that of the recurrent parental line. The persistence of a trait in the progeny, when the backcross parent exhibits an alternative character, may be considered evidence for plasmagene inheritance (Example 13.28).

EXAMPLE 13.24 In higher plants, pollen usually contributes very little, if any, cytoplasm to the zygote. Most of the cytoplasmic elements are transmitted through the maternal parent. In the plant called "four-o'clock" (Mirabilis jalapa), there may be normal green, pale green, and variegated branches due to two types of chloroplasts. Plants grown from seeds that developed on normal green branches (with all normal chloroplasts) will all be normal green; those that developed on pale-green branches (with abnormal chloroplasts) will all be pale green; those on variegated branches (with both normal and abnormal chloroplasts) will segregate green, pale green, and variegated in irregular ratios. The type of pollen used has no effect in this system. The irregularity of transmission from variegated branches is understandable if plasmagenes exist in the chloroplasts, because there is no mechanism to ensure the regular distribution of chloroplasts to daughter cells as there is for chromosomes.

EXAMPLE 13.25 The uniting gametes of the single celled alga Chlamydomonas reinhardi (Fig. 5-1) are morphologically indistinguishable. One strain of the alga that is streptomycin-resistant (sr) and of the "plus" mating type (mt+) is crossed to a cell of "negative" mating type (mt) that is streptomycin-sensitive (ss). All progeny are resistant, but the nuclear genes for mating type segregate as expected: 1/2mt+, 1/2mt. The reciprocal cross ss mt+ × sr mt, again shows the expected segregation for mating type, but all progeny are sensitive. Repeated backcrossings of sr mt+ to ss mt fail to show segregation for resistance. It appears as though the plasmagenes of the mt strain become lost in a zygote of mt+. The mechanism that inactivates the plasmagenes of mt in the zygote is not well understood.

EXAMPLE 13.26 Slow-growing yeast cells called petites lack normal activity of the respiratory enzyme cytochrome oxidase associated with the mitochondria. Petites can be maintained indefinitely in vegetative cultures through budding, but can sporulate only if crossed to wild type. When a haploid neutral petite cell fuses with a haploid wild-type cell of opposite mating type, a fertile wild-type diploid cell is produced. Under appropriate conditions, the diploid cell reproduces sexually (sporulates). The four ascospores of the ascus (Fig. 6-4) germinate into cells with a 1 : 1 mating-type ratio (as expected for nuclear genes), but they are all wild type. The petite trait never appears again, even after repeated backcrossings of both mating types to petite. The mitochrondrial factors for petite are able to perpetuate themselves vegetatively, but are "swamped," lost, or permanently altered in the presence of wild-type factors. Neutral petite behaves the same in reciprocal crosses regardless of mating type, and in this respect is different from the streptomycinresistance factors in Chlamydomonas (see Example 13.25).

EXAMPLE 13.27 Another type of petite in yeast, called suppressive, may segregate, but in a manner different from chromosomal genes. When haploid suppressive petites are crossed to wild types and each zygote is grown vegetatively as a diploid strain, both petites and wild types may appear, but in frequencies that are hardly Mendelian, varying from 1 to 99% petites. Diploid wild type cells may sporulate producing only wild-type ascospores. By special treatment, all diploid zygotes can be made to sporulate. The majority of the ascospores thus induced germinate into petite clones. Some asci have 4, 3, 2, 1, or 0 petite ascospores, suggesting that environmental factors may alter their segregation pattern. Nuclear genes, such as mating type, maintain a 1 : 1 ratio in all asci.

EXAMPLE 13.28 The protoperithecial parent in Neurospora (Fig. 6-5) supplies the bulk of the extrachromosomal material of the sexually produced ascospores. Very slow spore germination characterizes one strain of this fungus. The trait exhibits differences in reciprocal crosses and maternal inheritance, and fails to segregate at meiosis. When the slow strain acts as protoperithecial parent and the conidial strain has normal spore germination, all the progeny are slow, but possess 50% of the nuclear genes of the conidial parent. Each generation is then backcrossed to the conidial parent, so that the F2 contains 75%, F3 contains 87.5%, etc., of nuclear genes of the conidial parent. After the fifth or sixth backcross, the nuclear genes are almost wholly those of the conidial parent, but the slow germination trait persists in all of the progeny.

Exceptions are known to the generalization that cytoplasmic factors are maternally inherited.

EXAMPLE 13.29 When a green strain of geranium (Pelargonium) is crossed to a strain with white-margined leaves, the progeny may have green, white, or white-margined leaves. Since the results of reciprocal crosses are the same, it has been hypothesized that plastids can be transmitted to offspring by both the male and female gametes. Cells that contain a mixture of plastid genomes are said to be heteroplasmic; those that contain only one type of plastid genome are called homoplasmic.

Notice that the F1 is coiled dextrally, not because its own genotype is s+/s, but because the maternal parent possessed the dominant dextral gene s+. Likewise in the first selfing generation, all are phenotypically dextral regardless of their own genotype because the F1 was s+/s. In the second selfing generation, we expect the following:

Practice problems for these concepts can be found at:

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