Meiosis and Mitosis Essay example - 752 Words | Bartleby
The first chromosome study of rhododendrons was published by Dr. Karl Sax of the Arnold Arboretum in 1930. In a paper titled "Chromosome Stability in the Genus ," he determined the basic chromosome number† (x=13) as well as the existence of tetraploidy in and (13).
His most significant finding, however, was the frequent occurrence of normal meiosis in a wide array of interspecific F1 hybrids. Chromosomes from taxonomically divergent species exhibited few abnormalities in pairing and disjunction (separation) during meiosis in developing pollen cells. These observations led Dr. Sax to conclude that "there is complete or near complete compatibility of the parental chromosomes in the hybrids, although the parents [Asian and North American rhododendrons] have been separated for millions of years."
This conclusion has been generally substantiated by subsequent research and practical experience. A cytogenetic study of native azalea species found that the frequency of meiotic aberrations and pollen abortion in natural hybrids was low (11), although pollen abnormalities were reported to be more common in deciduous azaleas cultivars (18). In segregating rhododendron populations, however, Mendelian inheritance of isozyme markers was interpreted as evidence that normal meiosis occurs in interspecific hybrids with complex pedigrees (8).
Rhododendron hybridizers have benefited from this broad compatibility among species. Unlike breeders working with other plant genera where strong crossing barriers exist, they are not confined to within species (intraspecific) variability. For over a century they have been successfully recombining traits between species (interspecific). Many of the interspecific hybrids produced are fertile and have been used as parents to produce advanced generation offspring. These historical trends suggest that meiosis is stable in many of these hybrids.
Of course, not all rhododendron hybrids are fertile. This is increasingly true as one moves from intra- to inter-sectional and even wider crosses. Hybrids resulting from crosses between different subgenera, such as azaleodendrons, are often difficult to obtain, and when they do occur they are usually sterile (6). As a breeder interested in transferring traits via wide hybridizations, I have more than a passing curiosity in these infertility barriers. The research presented here represents an attempt to determine the biological mechanisms leading to hybrid sterility in rhododendrons.
A reasonable starting point was to extend Dr. Sax's conclusions to the phenomenon of hybrid sterility. If chromosome "stability" is characteristic of many fertile interspecific hybrids, then chromosome "instability" might be a likely cause of sterility. A chromosomal basis for hybrid sterility has been widely documented in other plant species (2), where structural differences between parental chromosomes interfere with normal pairing and disjunction during meiosis. To test this hypothesis, I examined meiosis in sterile F1 hybrids resulting from crosses between different subsections of the genus .
Mitosis vs Meiosis - Term Paper
Stages of Meiosis
Cells of higher organisms possess a basic chromosome number or ploidy (x), the smallest number of chromosomes in a chromosome set. Multiple sets of chromosomes are usually required in order for cells to function normally during the growth and development of plants and animals. Diploid organisms have 2 chromosome sets (2x), while polyploids have 3 or more sets - for example triploids (3x) and tetraploids (4x). Each chromosome in a set is distinct in physical and genetic structure. In diploids, identical chromosomes from each set are called homologous pairs or homologues.
When organisms reproduce, they form specialized sex cells or gametes in which the normal chromosome number is halved by the process of meiosis. The reduction in ploidy is accomplished by cell divisions. In the case of diploid plants, pollen and egg cells are haploid and contain a single set of chromosomes. Upon fertilization, the diploid condition is restored to the embryo and the process is repeated in subsequent generations.
Most of the stages of meiosis are outlined below in a hypothetical diploid organism having a basic chromosome number x = 2. The chromosomes are distinguishable by length; one is short, the other long. The fundamental events in meiosis are 1) pairing of homologous of chromosomes 2) physical exchange of genetic material and 3) subsequent separation of homologues and reduction of cell ploidy level by one half.
Results and Discussion
Lack of pollen staining indicated that nearly all the male gametes produced by 'Tow Head', 'Jericho', and 'Senegal' were aborted even though their parents had moderate to high levels of pollen viability (Table 1). Siblings of 'Jericho' and 'Senegal' also showed high rates of pollen abortion (> 95%), suggesting that hybrid sterility was a typical result from these particular subsectional crosses.
Pollen staining often overestimates viability when compared to and germination tests (14, 17), and so it is reasonable to conclude that these clones have extremely low levels of male fertility. Although conclusive data are lacking, it is quite likely that these clones are female sterile as well. One cultivar, 'Tow Head', has failed as a seed parent in several controlled crosses at the Leach Research Station, and none of the three cultivars has set open-pollinated seed capsules in the past two years.
The cultivar 'Jericho' is registered as a hybrid between diploid Mt Kuromi form, and 'Epoch', the tetraploid form of synthesized by Dr. Kehr (5). I included it in this study because crosses between diploids and tetraploids often generate sterile triploid progeny. However, chromosome counts in both mitotic and meiotic cells indicated that 'Jericho' is a diploid (Fig. 1A, 1G). Hybrid verification using isozyme markers (enzyme variants) further demonstrated that 'Jericho' carries genes attributable to the seed parent but not to 'Epoch' (Fig. 2). There is therefore little doubt that 'Jericho' results from an open-pollinated seedling from . The real father may be one of the numerous diploid clones that are in close proximity to, and bloom concurrently with, at the Research Station. If this is the case, then 'Jericho' is, for the purposes of this study, fairly similar to 'Senegal' in its genetic composition.