Chapter 5 Genetic Analysis of Apomixis - cimmyt

Apo";xn ood ,~. M""ogemeo' of 60..,;, Di..,,~y 13Polyploid forms are autonomous apomicts,either facultative or obligate. Fruits(propagules) can be obtained withoutpollination, after eliminating anthers andstigmas (Mogie and Ford 1988).In Parthenillm (Asteraceae), diploid forms with2n = 2x = 36 are sexual, and polyploid formswith 211 = 54, 72, 90, or 108, are apomictic. Inthis genus, pseudogamy is prevalent andtherefore fruits are not produced in the absenceof pollen (Powers and Rollin 1945). Ploidybuildup occurs through production of 211 + 11progeny (Powers and Rollins 1945), andproduction of haplOids from hexaploids hasbeen documented (Powers 1945). In this case,a cycle exists between tetraploids, hexaploids,and triploids, with a possibility ofincorporating diploid forms into the cyclethrough their production of 211 + 11 progenywith 54 chromosomes.Capillipediunr DichanthiunrBothriochloaAgamic Complex (Poaceae)The genera of Capi/lipedium, Dichanthium, al1dBothriochloa are distributed over Europe, theMediterranean region, Asia, Australia, and theNew World, and have been studied in detailby Harlan, de Wet, and coworkers. De Wet(1968) described a possible evolution in thegenus Dichanthium based on ploidy cyclesinvolving diploids, tetraploids, and haploids.In a broader approach, de Wet and Harlan(1970) described the interrelationshipsbetween species of the three genera of thisagamic complex (Figure 2.1). The mostcommon ploidy levels are 2x, 4x, 6x, as well assome pentaploid forms. Diploids are sexual,and polyploids are apomictic. However, formsfrom the New World are sexual and polyploid.Triploid forms are not mentioned. Gene flowoccurs in several directions, but in some casesis limited by incompatibility barriers. Geneticexchanges between Capillipedium andDiclulI1thium are effective only when speciesof Bothriochloa are involved as genetic bridges.Haploid production was detectedexperimentally and haploid plants werefound to be either sexual or sterile. Tetraploidplants can be recovered from thesedihaploids through the formation of 2n + nprogeny, with n proceeding from pollen oftetraploid plants. Rates of 2n + n productionof up to 15% have been observed.Panicum maximum Agamic Complex(Poaceae)"Guineagrass" has its origin in East Africa.It has colonized West Africa as well as thetropical areas of the New World. This agamiccomplex includes three species: Panicummaximum, P. trichocladum, and P. infestum(Combes 1975). Panicum maximum is Widelydistributed and sexual diploid forms havebeen identified (Combes and Pemes 1970),though they are very rare, having only beenfound in three very limited areas in Tanzania(Combes and Pemes 1970; Nakajima et a!.1979). The other forms are tetraploid andfacultative apomicts. Occasionally, pentaandhexaploid forms have been detected.ICAPILLIPEDIUM II DICHANTHIUM Iorillolum(2x, 4x)popillolum(6x)porvinorum olsimile(2x, 4x) (2x, 4x) cori!um ontorum~ (2x,4x) 1J{2x,4x)---~:~;;~:)~-::~~==-~~~~-:~~~h;~2:~;~----,fl '17, ,,,!~,~ 'u ~' 4ewortiono inlermedio (4x) ischoemum(5x,6x) / 11 ~'4~ (4x,6x)conconenlis / insculpro (5x, 6xl waodrowii (5x)15X)'/ ~ "­/ t>