INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

APOMIXIS

Fehr, Chapt. 2
Briggs and Knowles, Chapt. 4
Hybridization of Crop Plants, Chapt. 3.


Apomixis: A type of reproduction in which sexual organs or related structures take part, but in which the seeds are formed without the union of gametes.
Apogamy: Embryo develops from two haploid nuclei other than the egg. (Progeny may be different from parent.)
Apospory: Embryo sac is formed directly from a somatic cell without reduction and formation of spores. (Progeny is identical to parent.)
Diplospory: Embryo develops directly from the megaspore mother cell without reduction. (Progeny is identical to parent.)
Parthenogenesis: Development of an egg (reduced or unreduced egg) cell into an embryo without fertilization. Also includes situation where have haploid (n) plant formation. (Progeny may be different from the parent).
Pseudogamy: Pollen necessary as stimulant. Presence of pollen tube necessary and/or actual fertilization necessary for the embryo to develop. This requirement often misleads the interpretation of whether or not the seed is formed by self/cross pollination or apomixis.

Consider how the parent relates to the progeny in the various types of Apomixis.

There is also facultative (inducible or controlled) apomixis. This is often used in grass breeding.

Note the difficulties in using a sexual process to introduce variation, but then remove the sexual process to maintain the genetic combination.

  • Why are we so interested in apomixis?


    1. Characteristic of a number of different crop species.


    2. If present, then might be misled in interpreting progeny evaluations particularly if one thinks one has made a cross.


    3. Need to consider a mechanism for generating variability especially in case of an obligate apomict.


    Advantage of maintaining desirable heterozygous individual.

  • Progeny Comparison of Self-pollinated, Cross-pollinated, and Apomictic Plants:


    1. Self-pollinated plant progeny will be phenotypically similar to their parents. If you select seed from differing lines within a self-pollinated crop population, the progeny will reflect the differences among the parent selections. Except in early generation selections or stable heterogeneous populations, the progeny lines should be uniform. Most, if not all variation is between lines (as opposed to within a line) within a population.


    2. Cross-pollinated plant progeny will phenotypically dissimilar to the parent unless the parent population is genetically homozygous. Variation is between and within lines derived from the population.


    3. Apomictic progeny, depending upon the type of apomixis, will be uniform with the parent. Variation will be between lines and not within lines. If unfertilized egg forms seed, the progeny may be haploid or dissimilar from the parent. However, its progeny will be uniform.


    SUMMARY:

    Remember that the pollination mechanism will often determine the type of plant breeding method that you are using. For example, self pollinated crops are often difficult to hybridize, but very easy to self. Hence, breeding methodologies relying on selfing are usually used (inbreeding). Cross pollinated crops are readily cross pollinated, hence, breeding methods that rely upon open pollinations are often used as opposed to selfing. Note that there are advantages to open pollinations (namely, increased gene recombination and reduced gene fixation). With the advent of continued research on both self and cross pollinated breeding techniques highlighting the advantages of each, more and more the breeding methodologies as determined by the pollination mechanism are becoming less distinct or exclusive of each other.

    Heterosis: hybrid vigor or advantage such that an F1 hybrid falls outside the range of the parents with respect to some character or characters. Usually related to size, rate of growth, or general vigor. Heterosis is usually higher in cross pollinated crops than in self pollinated crops. Inbreeding depression is usually smaller in self pollinated crops than in cross pollinated crops.