Principles of Plant Genetics and Breeding. George Acquaah

Figure 5.11 The three systems of cytoplasmic genetic male sterility. The three factors involved in CMS are the normal cytoplasm (N), the male sterile cytoplasm (S), and the fertility restorer (Rf, rf).

       Exploiting male sterility in breeding

      Male sterility is used primarily as a tool in plant breeding to eliminate emasculation in hybridization. Hybrid breeding of self‐pollinated species is tedious and time‐consuming. Plant breeders use male‐sterile cultivars as female parents in a cross without emasculation. Male‐sterile lines can be developed by backcrossing.

      Using genetic male sterility in plant breeding is problematic because it is not possible to produce a pure population of male‐sterile plants using conventional methods. It is difficult to eliminate the female population, before either harvesting or sorting harvested seed. Consequently, this system of pollination control is not widely used for commercial hybrid seed production. On the contrary, CMS is used routinely in hybrid seed production in corn, sorghum, sunflower, and sugar beet. The application of male sterility in commercial plant hybridization is discussed in Chapter 19.

       Dichogamy

      Dichogamy is the maturing of pistils and stamens of a flower at different times. When this occurs in a self‐pollinated species, opportunities for self‐pollination are drastically reduced or eliminated altogether, thus making the plant practically cross‐pollinated. There are two forms of dichogamy: protogyny (stigma is receptive before the anther is mature to release the pollen) and protandry (pollen is released from the anther before the female is receptive).

      5.5.3 Genetic and breeding implications of autogamy

Self‐pollination is considered the highest degree of inbreeding a plant can achieve. It promotes homozygosity of all gene loci and traits of the sporophyte. Consequently, should there be cross‐pollination the resulting heterozygosity is rapidly eliminated. To be classified as self‐pollinated, cross‐pollination should not exceed 4%. The genotypes of gametes of a single plant are all the same. Further, the progeny of a single plant is homogeneous. A population of self‐pollinated species in effect comprises a mixture of homozygous lines. Self‐pollination restricts the creation of new gene combinations (no introgression of new genes through hybridization). New genes may arise through mutation, but such a change is restricted to individual lines or the progenies of the mutated plant. The proportions of different genotypes, not the presence of newly introduced types, define the variability in a self‐pollinated species. Another genetic consequence of self‐pollination is that mutations (which are usually recessive) are readily exposed through homozygosity, for the breeder or nature to apply the appropriate selection pressure on (see Box 5.1).

Industry highlight boxes Introgression breeding on tomatoes for resistance to powdery mildew

      Yuling Bai

      Wageningen UR Plant Breeding, Droevendaalsesteeg 1, 6708 PB, Wageningen, The Netherlands

       Tomato and its wild relatives

      Tomato (Solanum lycopersicum) is very important vegetable both for the fresh market and for the processed food industry. Although cultivated as an annual, tomato grows as a perennial in its original habitat in Peru (Picken et al. 1985). The original site of domestication of tomato is likely in Mexico (Taylor 1986).

According the recent classification, tomato belongs to section Lycopersicon and has 12 wild relatives (Table B5.1). Of these 12 relatives, nine (no. 1 to 9 in Table B5.1) are previously defined in the genus of Lycopersicon (refer to as old Lycopersicon species). Accessions of nearly all these nine species have been successfully used to introduce valuable traits for crop improvement, especially monogenic sources conferring resistance to fungal, nematode, bacterial, and viral diseases. (Kiss, L. and Takamatsu, S. 2005). The phelogenic relation of these old Lycopersicon species with cultivated tomato has been extensively studied, based on comparative analysis of morphology, self‐compatibility, crossability, and molecular markers. The classical taxonomic characters which have been used to divide old Lycopersicon species are fruit color and self‐compatibility. In the phylogeny generated with molecular markers, different patterns of species relationships have been obtained, some of which are congruent with results of classical taxonomy and others add resolution to new divisions that are always in agreement. In general, we could conclude the following (i) species (S. lycopersicum, S. lycopersicum var cerasiforme, Solanum cheesmaniae, Solanum pimpinellifolium) with self‐compatibility and red fruits; (ii) Solanum peruvianum and Solanum chilense (green fruits and self‐incompatible) are closely related species; and (iii) species with green fruits, including Solanum chmielewskii, Solanum neorikii, Solanum habrochaites, and Solanum pennellii, have varied relationships with the rest depending on markers used for phylogeny.

Table B5.1 Old and new names of tomato and its wild relatives.

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No.	New Solanum name	Lycopersicon equivalent	Fruit color	Self‐compatibility	Ability to be crossed with other Solanum species	Section name within Solanum
1	Solanum lycopersicum	Lycopersicon esculentum	Red	Self‐compatible	Old “esculentum” group, crossable among these species, although it is sometimes only possible in one direction to make crosses	Section Lycopersicon
2	S. pimpinellifolium	L. pimpinellifolium	Red
3	S. cheesmaniae	L. cheesmaniae	yellow
4	S. chmielewskii	L. chmeilewskii	Green
5	S. neorickii	L. parviflorum	Green
6	S. habrochaites	L. hirsutum	Green