Chapter 5 Genetic Analysis of Apomixis - cimmyt

simultaneous division of the proximate eggand sperm nuclei in the zygote (Turcotte andFeaster 1973,1974).Autonomous parthenogenesis is the formationof embryo and endosperm withoutfertilization ofeither the egg or the central cell;such apomicts require no pollination for seedset and frequently are male-sterile.Autonomous endosperm formation has alsobeen reported in a case of adventitiousembryony, namely Euphorbia dulcis(Gustafsson 1946).2) Endosperm and embryo development.Pseudogamy and autonomous parthenogenesiscan be subdivided on the basis of theircapacity to form embryos in the absence ofendosperm. The apomictic egg dividesautonomously in Potentilla (pseudogamy) andWikstroemia (autonomous parthenogenesis)(Gustafsson 1946). The egg divides after thefirst endosperm nucleus in Themeda(pseudogamy) and Crepis (autonomousparthenogenesis), although the potential forits eventual division in the absence ofendosperm has not been critically evaluatedin most cases. In semigamy, the unfertilizedegg degenerates without dividing (Crane1978), contrary to an earlier report of eventualdivision (Cae 1953). There is also variation inthe fusion of the polar nuclei with each otherin apomicts. In autonomous parthenogenesis,the polar nuclei can fuse before dividing (e.g.,in Taraxacum, Elatostema, and most Alchemillaspecies), facultatively fuse (e.g., in Crepis), ordivide without fusing (e.g., in Antennaria)(Gustafsson 1946). In pseudogamy, the polarnuclei may (Ranunculus auricomus; Nogler1972) or may not (Amelanchier; Campbell et al.1987) fuse with each other when the centralcell is fertilized. Both polar nuclei contributeto the endosperm in semigamous Cooperin andin the Se mutant of cotton (Crane 1978; G. L.Hodnett, unpublished).Alternative ClassificationsThe standard classification of apomicticembryo sacs (e.g., Asker and ]erling 1992) hasfollowed Edman (1931) and Gustafsson (1946)in recognizing embryo sacs that arise fromdiploid megaspores (diplospory) and embryosacs that do not arise from megaspores (apospory).Under this scheme, the Hieracium- andPanicum-types are aposporous, and theremaining seven types are diplosporous (Note:Types that were undescribed at the time ofthese old papers (Eragrostis, Panicum, Elymus,and Allium odorum) have been assigned on thebasis of the classifying paper's underlyingconcepts). Fagerlind (1940, 1944) defineddiplospory to be the production of diploidmegaspores from meiosis, in effect restrictingit to the Taraxacum-, Ixeris-, and Alliumodorum-types. He termed purely mitoticdevelopments apospory, which could begenerative (Antennaria, Eragrostis) or somatic(Hieracium, Panicum). The Blumea- and dyadformingElymus-types felI into a halfwaycategory, semiapospory. Battaglia (1963) alsorecognized apospory for purely mitoticdevelopments and differentiated gonial andsomatic subtypes. However, he grouped allmodifications of meiosis under aneuspory.Thus he considered the AlIium odorum-,Taraxacum-, Ixeris-, Blumea-, and dyadformingElymus-types to be fundamentallysimilar in that they involved modified meiosis.Conseq'uently, one issue that has emerged isthe definition of a megaspore: is it morefundamentalIy the product of amegasporocyte or the product of femalemeiosis? Many other terms have been definedand redefined, leading to terminological andconceptualconfusion. Forexample,]ohri et al.(1992) speak of "apospory of the Taraxacumtype"in Balanophora globosa. The majorproblem has been an absence ofhard evidenceas to what genetic changes are necessary toestablish apomixis and what the relevant geneproducts do. Furthermore, the traditional