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Apomixis in hawkweed: Mendel's experimental nemesis
Anna M G Koltunow1, Susan D Johnson, Takashi Okada
1CSIRO Plant Industry, Waite Campus, Glen Osmond, South Australia 5064, Australia. anna.koltunow@csiro.au
Journal of Experimental Botany
|February 22, 2011
Summary
Hawkweeds reproduce asexually through apomixis, bypassing meiosis and fertilization. Key genetic loci control this process, with sexual reproduction being the default mode in these plants.
Area of Science:
- Plant reproductive biology
- Genetics of apomixis
Background:
- Mendel's laws of inheritance were established using Pisum, but trait segregation was not observed in hawkweeds.
- Hawkweeds (Hieracium subgenus Pilosella) often reproduce asexually via apomixis, where seeds develop without meiosis or fertilization.
Purpose of the Study:
- To investigate the genetic mechanisms underlying apomixis in hawkweeds.
- To understand the role of specific genetic loci in controlling the switch between sexual and asexual reproduction.
Main Methods:
- Analysis of apomixis in Hieracium subgenus Pilosella.
- Investigating the function of dominant loci, LOSS OF APOMEIOSIS and LOSS OF PARTHENOGENESIS.
Main Results:
- Apomixis in hawkweeds involves mitotic embryo sacs formed by apospory, initiated by sexual reproduction cues.
- The dominant loci LOSS OF APOMEIOSIS and LOSS OF PARTHENOGENESIS regulate apomixis, enabling asexual seed development.
- Deletion or loss of function in these loci leads to reversion towards sexual reproduction.
Conclusions:
- Sexual reproduction is the fundamental mode in these apomicts, with apomixis superimposed.
- The identified loci likely facilitate apomixis by interacting with the sexual pathway rather than being essential for it.
- Facultative apomicts can produce rare sexual offspring due to incomplete penetrance of these dominant loci.
Related Concept Videos
01:43Incomplete Dominance
Gregor Mendel's work (1822 - 1884) was primarily focused on pea plants. Through his initial experiments, he determined that every gene in a diploid cell has two variants called alleles inherited from each parent. He suggested that amongst these two alleles, one allele is dominant in character and the other recessive. The combination of alleles determines the phenotype of a gene in an organism.
02:03Law of Independent Assortment
While Mendel’s Law of Segregation states that the two alleles for one gene are separated into different gametes, a different question of how different genes are inherited remains. For example, is the gene for tall plants inherited with the gene for green peas? Mendel asked this question by experimenting with a dihybrid cross; a cross in which both parents are homozygous for two distinct traits resulting in an F1 generation that are heterozygous for both traits.
01:20Monohybrid Crosses
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01:49Law of Segregation
When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F1 generation, and could reappear in the next generation (F2). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F1 generation. When he crossed the F1 plants, he found that 75% of the offspring in the F2 generation had the dominant phenotype, while 25% had the recessive phenotype.
01:18Dihybrid Crosses
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01:29Nondisjunction
During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.