Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 306

Hybridization is often conducted routinely without any problems when individuals from the same species are involved, provided there are no fertility regulating mechanisms operating. Even when such mechanisms exist, hybridization can be successfully conducted by providing appropriate pollen sources. Sometimes, plant breeders are compelled to introduce desired genes from distant relatives or other more or less related species. Crossing plants from two different species or sometimes even plants from two genera is challenging and has limited success. Often, the breeder needs to use additional techniques (e.g. embryo rescue) to intervene at some point in the process in order to obtain a mature hybrid plant. Reproductive isolation barriers may be classified into three categories (Table 6.2). These barriers maintain the genetic integrity of the species by excluding gene transfer from outside species. Some barriers occur before fertilization, some after fertilization. These barriers vary in degree of difficulty to overcome through breeding manipulations.

 Spatial isolationSpatial isolation mechanisms are usually easy to overcome. Plants that have been geographically isolated may differ only in photoperiod response. In this case, the breeder can cross the plants under a controlled environment (e.g. greenhouse) by manipulating the growing environment to provide the proper duration of day length needed to induce flowering.

 Pre‐fertilization reproductive barrierThese barriers occur between parents in a cross. Crops such as wheat have different types that are ecologically isolated. There are spring wheat types and winter wheat types. Flowering can be synchronized between the two groups by, for example, vernalization (a cold temperature treatment that exposes plants to about 3–4 °C) of the winter wheat to induce flowering (normally accomplished by exposure to the winter conditions). Mechanical isolation may take the form of differences in floral morphology that prohibit the same pollinating agent (insect) from pollinating different species. A more serious barrier to gene transfer is gametic incompatibility, whereby fertilization is prevented. This mechanism is a kind of self‐incompatibility (see Chapter 4). The mechanism is controlled by a complex of multiple allelic system of S‐genes that prohibit gametic union. The breeder has no control over this barrier.

 Post‐fertilization reproductive barriersThese barriers occur between hybrids. After fertilization, various hindrances to proper development of the embryo (hybrid) may arise, sometimes resulting in abortion of the embryo, or even formation of a haploid (rather than a diploid). The breeder may use embryo rescue techniques to remove the embryo and culture it to a full plant. Should the embryo develop naturally, the resulting plant may be unusable as a parent in future breeding endeavors because of a condition called hybrid weakness. This condition is caused by factors such as disharmony between the united genomes. Some hybrid plants may fail to flower because of hybrid sterility (F1 sterility) resulting from meiotic abnormalities. On some occasions, the hybrid weakness and infertility manifest in the F2 and later generations (called hybrid breakdown).

Table 6.2 A summary of the reproductive isolation barriers in plants as first described by G.L. Stebbins.

External barriersSpatial isolation mechanisms: associated with geographic distances between two speciesPre‐fertilization reproductive barriers: prevents union of gametes. Includes ecological isolation (e.g. spring and winter varieties), mechanical isolation (differences in floral structures), and gametic incompatibility. Internal barriersPost‐fertilization reproductive barriers: leads to abnormalities following fertilization (hybrid inviability or weakness and sterility of plants).