Chapter 5 Genetic Analysis of Apomixis - cimmyt

14 JoG.. B.rth""dTable 2.8 Distribution of clones according to ploidylevel from the P. maximum collection established inCote d'ivoire (Combes 1975)Total 2x 3x* 4x 5x** 6x**551 19 0 506 12 13• 3x ploidy level not found in wild populations•• 5x and 6x overrepresenled in Ihis collection (from over·collecting inthese populotions)Table 2.8 shows the distribution of clonesaccording to their ploidy level in a collectionestablished in Cote d'Ivoire.Triploid plants have been experimentallyobtained from hexaploids (n + 0 progeny) aswell as from diploid x tetraploid crosses.(Poly-) haploids also have beenexperimentally obtained from tetraploids,and the resultant plants have been eithersexual or sterile (potentially apomictic asshown by embryo sac analyses). Thesefindings led Savidan and Pernes (1982) topropose an evolutionary scheme based onploidy cycles involving di-tetra-haploid levelsas in the Dichanthium complex. The changefrom diploid to tetraploid is realized through2n + n hybridization with pollen fromtetraploid plants. In this system, sexuality ismaintained at the diploid level. Contactbetween diploid and tetraploid plants allowsgenetic exchange between these pools(compartments) and creation of sexualtetraploid plants, allowing the release of newgenetic diversity at the tetraploid level.Paspalum Agamic Complex (Poaceae)The center of diversity for the genus Paspalumis in South America. Studies conducted byQuarin (1992), Norrman et al. (1989) andcollaborators at the Instituto de Botanica delNordeste, Corrientes, Argentina (IBONE)show that many species in this genus havegenetic pools at two or more ploidy levels(Table 2.9).ln the pool with the lowest ploidylevel, plants are sexual and self-incompatible,while in pools with higher ploidy levels, theyare apomictic and self-compatible. In manycases, the two pools are at the diploid andtetraploid level (group 3 ofTable 2.9). In somespecies, however, the sexually selfincompatibleplants are tetraploid and the selfcompatibleapomictic plants are hexaploid oroctoploid (group 6 ofTable 2.9).Some species that are sexual and selfcompatibleat the tetraploid or hexaploid level(groups 5 and 7 of Table 2.9) are not apomicticat higher ploidy levels. In other species,triploids are often apomictic and are found inspecies with sexual diploids and apomictictetraploids.As with previously cited agamic complexes,sexual forms are found at the lowest ploidylevel and apomictic forms at the other levels.However, in this example, the relationshipincludes the incompatibility system.Apomictic plants are self-eompatible and thecorresponding sexual plants are selfincompatible.Experiments should beconducted to determine whether this alsooccurs in other agamic complexes.Tripsacum Agamic Complex (Poaceae)The Tripsacum genus is restricted to the NewWorld, from 42°N to 240$. Its center ofdiversity(or origin) is located in Mexico and Guatemala,and 11 of the 16 species described for the genusare found in this region. These 11 species showdifferent ploidy levels both within and amongthemselves. The collection the team assembledfrom Mexico displayed the followingdistribution (unit = one ploidy level of onespecies in one population): diploids, 16.4%;triploids, 7.9%; tetraploids, 72%; penta- andhexaploids, 3.7%.When compared to other agamic complexes,a high frequency of triploid plants in theTripsawm complex was observed. These wildtriploid plants are apomictic, produce fertilepollen, and set good seed. All of the naturalpolyploids we observed were apomictic(Leblanc et al. 1995; and unpublished data).