Chapter 5 Genetic Analysis of Apomixis - cimmyt

158 Yv•• Savid..unreduced polar nuclei. This trait can also beused for screening modes of reproduction insegregating populations by means of flowcytometry (Grimanelli et al. 1997). Previousstudies showing that 5-6 backcrosses areneeded to produce introgressed 20­chromosome maize plants provided anotheradvantage to using this species. Disadvantagesinclude total male sterility, which is seeminglyretained until reaching addition forms withvery few Tripsacum chromosomes, and thedifference in basic chromosome numbers (x =18 compared to x = 10 in maize).Production of Interspecific orIntergeneric F 1HybridsSeveral procedures are available to producehybrids between cultivated and distantlyrelated wild species. Special techniques,including chromosome manipulation,bridging species, hormonal treatment, embryorescue, ovary culture, and in vitro pollination,are available for overcoming the crossincompatibility and the sterility of the F1s. Thepresence of apomixis makes the cross moredifficult because it can only be performed inone direction, with the apomixis progenitorbeing used as pollinator. Therefore, the donormust exhibit good pollen fertility. Because mostapomicts require fertilization with reducedpollen to produce endosperm, pollen qualityis generally not affected by apomixis. Anexception to this rule is Elymus rectisetus, inwhich male infertility is a problem with mostaccessions O. G. Carman, personal comm.).Crossing TechniquesMost of the crossing techniques are commonto intra- and interspecific crosses. Aprerequisite is good knowledge of the selfsterilityor self-incompatibilitysystems existingwithin the crop. For most crops, however, handemasculation is preferred.Crossing species with different flower sizesand shapes may require special tricks, e.g., inthe case of maize x Tripsacum, more hybridsare produced if the silks are shortened to about2-3 cm. Most wide crosses require embryorescue techniques, using classical media suchas MS (Murashige and Skoog 1962) or N 6(Chuet al. 1975). Small embryos from maize xTripsacum F1s grew better on 50 gil sucrose ascompared with standard embryo culturemedium containing 30 gil sucrose. Severalenvironmental factors can further affect theproduction and culture of hybrid embryos. Asa result, the production of hybrids may be goodone year, but poor the next.When apomixis is not found in wild relatives,transfer may be attempted from a more distantapomictic species by using protoplast fusion.Such a transfer was started for sorghum usingapomixis from Cenchrus ciliaris (Bharathi et al.1991). However, no reports of plantregeneration have surfaced to date, apomicticor not, from such protoplast fusions. A morerecent approach, developed by Ramulu et al.(1996), explores the production ofmicroprotoplasts containing only one or twoalien chromosomes and the direct productionof monosomic addition lines after fusion withprotoplasts from the receptor species.Sterility of the FISSterility in interspecific and intergeneric F1sand supsequent backcross generations is acharacteristic of wide crosses. Restoringfertility of the F] hybrids through chromosomedoubling is the most common approach. Inboth pearl millet and maize transfer attempts,however, F1s from some wild speciesaccessions were totally sterile, while thoseobtained from other accessions showed somedegree of fertility, making the chromosomedoubling unnecessary.The transfer programs in pearl millet andwheat have produced F 1 hybrids with some