Chapter 5 Genetic Analysis of Apomixis - cimmyt

96 10k. G.(o,.,..mechanisms (Figure 7.1). For example,Antennaria-type diplospory is identical totetraspory (sexual), except that the nucleardivisions leading to a tetranucleate embryo sacare meiotic in tetraspory but mitotic inAntennaria-type diplospory. Both Antennariatypediplospory and tetraspory occur inAntennaria, Erigeron, Limonium, and Rudbeckia(Carman 1997). In both anomalies, MMC lackcallose (Peel et a1. 1997a). Ixeris-typediplospory is even more similar to tetrasporybecause, as in tetraspory, a meiotic prophaseoccurs. However, in the diplosporousmechanism, a first division restitution ensues.Ixeris-type diplospory is identical to bispory(sexual) except that meiosis I fails in the former(Figure 7.1). Both Ixeris-type diplospory andbispory (and apospory and tetraspory) occurin Erigeron. Allium odorum-type diplosporyand bispory differ only in that a chromosomalendoreduplication occurs in the former. Bothare found in Allium. The recognition of thesedevelopmental similarities and phylogeneticassociations led Carman (1997) to analyzetaxonomic data for all known species thatexpress these anomalies. It was found thatapomictic, polysporic, and polyembryonicspecies are polyphyletic and tend to bephylogenetically related. Many highlysignificant associations were discovered.The variation represented in Figure 7.1 issomewhat continuous. For example,Antennaria-type diplospory is similar to bothtetraspory and apospory. In some families,both species and genera span this continuum.For example, apomixis and polyspory occurtogether in 13 of 127 apomixis-containinggenera (Allium, Antennaria, Burmannia, Cordia,Cynoglossum, Erigeron, Eurybiopsis, Leontodon,Limonium, Rubus, Rudbeckia, Sambucus, andTridax) and 18 of 33 apomixis-containingAbundantcollole IynlhelilUni-nudeote embryo lOCIFigure 7.1 Developmental stages during megasporogenesis and embryo-sac development in selected sexual(monosporic, bisporic, and tetrasporic) and apomictic (Allium odorum-type diplospory, Antennaria-typediplospory, Taraxacum-type diplospory, Ixeris-type diplospory, Blumea-type diplospory, and apospory)angiosperms.