Chapter 5 Genetic Analysis of Apomixis - cimmyt

106 J.hG.eforseveral thousand to a hundred thousandyears each. Likewise, most of the major glacialevents consisted of glacial advancesinterrupted by minor interglacial periods thatlasted for a few thousand years (Frakes et al.1992). Hence, during the Pleistocene, the highlatitudes of both hemispheres were repeatedlydeglaciated and revegetated by cosmopolitantaxa capable of adapting to cool climates, longdays, and short growing seasons.A long-day flowering response and aprecocious meiosis and embryo-sacdevelopment in young ovules of sexualAnlennaria, and probably many other taxa, areadaptations to short summers in high latitudesor altitudes (Carman, unpublished). Glacialadvances, which followed the numerousinterglacial periods, cooled the midlatitudes,permitting higher latitude flora to invademidlatitude flora. This provided opportunitiesfor ecotypes with a putatively-precociousfemale development (from higher latitudes orelevations, i.e., temperate to arctic climates) toform polyploids with ecotypes (or relatedspecies) with a putatively-delayed femaledevelopment (from lower latitudes, i.e,. tropicto full-sun temperate climates). Suchpolyploids may have given rise to modernapomicts (Carman 1997, 2000).This interpretation is consistent with theobserved effects of climatic factors onfacultativeness in certain apomicts. Forexample, exposing Dicnnlhillm a11l1ll11111lm, D.illiermedillm, and Themedn lrumdm to short daysand low temperatures increases the frequencyof apospory. The opposite conditions increasethe frequency of sexuality in a partially-sexualDicnllihilim hybrid (reviewed by Asker andJerling 1992; Carman 2000). Such shifts infacultativeness are expected if signaltransduction pathways vary among genomesin sensitivity to morphogens (hormones, etc.)that accumulate in response to changingseasons and photoperiods.Once asynchrony is initiated, further shifts infacultativeness might occur in response togrowing conditions. G. Ledyard Stebbins(personal communication, 1997) suggests thatconditions favoring rapid growth (hightemperatures, moisture, and light) mightenforce competition between asynchronousgenomes causing an increased frequency ofapomixis. This prediction was observed inclones of F 1hybrids obtained between wheatand diplosporous Elymlls recliselus. Clonesgrown under favorable conditions (hightemperatures and light intensities) producedmore apomeiotic-like MMCs (Peel et al. 1997b).Additional research should be conducted toelucidate such effects on facultativeness (Askerand Jerling 1992). Such research could provideclues concerning the nature of the divergentparental phenotypes that may have producedapomicts upon hybridization and polyploidizationduring the Pleistocene.The HFA theory also suggests that the role ofapomixis in evolution is more prominent thanpreviously supposed. What happens if someof the many checkpoint g-enes permitting areasonably smooth replacement ofdevelopmental segments (resulting inapomixis) are mutated or lost duringdiploidization? Phylogenetic and genomicevidence suggests that such confusions ofdevelopment may manifest themselves aspolysp~ry or polyembryony. Thus, apomixis,instead of being an evolutionary dead end,may occasionally serve as an evolutionaryspringboard in the evolution of normal ordevelopmentally-novel paleopolyploid sexualspecies and genera (Carman 1997).Mendelian Analyses of ApomixisEssentially all Mendelian analyses of apomixisface reinterpretation if the HFA theory iscorrect. Data from a variety of apomicts haveon one or more occasions (or when grown incertain environments) tended to fit atetrasomic inheritance model with apomixis