Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 3Classification of Apomictic MechanismsCHARLES F. CRANEIntroductionApomixis has traditionally beenseparated intoasexual seed production (agamospermy) andreplacement of flowers by vegetativepropagules (vivipary). In practice, manyresearchers define apomixis as agamospermy,which in turn is divided into adventitiousembryony and gametophytic apomixis.Adventitious embryony is the formation ofsomatic embryos from ovular tissues outsidethe embryo sac, although endosperm in theembryo sac usually is necessary to supporttheir maturation, and the resulting somaticembryos sometimes compete with a zygoticembryo within the same ovule. Gametophyticapomixis is, at the least, a two-step processinvolving the production of a 2nmegagametophyte, whose egg developswithout fusion of egg and sperm nuclei; otheraspects of fertilization can be completelynormal or completely missing, depending onthe type.Adventitious embryony can be divided intonucellar and integumentary embryony,depending on where the embryos arise.According to Naumova (1993), adventitiousembryos in nature always originate fromsingle cells, termed embryocytes, which candifferentiate in the nucellus or integuments,before or after fertilization of the embryo sac.As summarized by Naumova (1993), the earlyorganization of adventitious embryoscharacteristically differs from that of zygoticembryos, possibly because space isconstrained and the mechanical stress field inthe nucellus differs from that in the embryosac. Multiple adventitious embryos candevelop asynchronously within the sameovule (Gustafsson 1946), whereuponcompetition and packing further affect theirmorphology. Polyembryony is very frequentin most cases of adventitious embryony, andfertilization of the central cell is generallynecessary for seed set. The developmentalinterpretation of adventitious embryony issimple: the embryocytes are induced to act likezygotes. The number of ways in which thisinduction can occur is not known, but they allmust cause repeated mitosis.Types of GametophyticApomixisIn gametophytic apomixis, 2n embryosacs canarise in at least nine different ways, dependingon the species. The embryo and endospermin such embryo sacs can develop in at leastfive ways. Therefore, at least 45 types ofgametophytic apomixis are theoreticallypossible, but only half of these account fornearly all cases of naturally occurringgametophytic apomixis. The nine types areschematically diagrammed in Figure 3.1,which lays out their proposed developmentalbasis. The types are described below(references are given afterward as part of ahistorical perspective). Note that "strike"(nondivision or precocious degeneration ofnuclei in the chalazal end of developing