Asynchronous replication ofheterochromatin inmaize

Replication patterns offour classes ofhetero- chromatin inthechromosomes ofmaize wereanalyzed. Cen- tromeric heterochromatin, distal heterochromatin oftheB chromosomes, andknobheterochromatin all haveasynchronous late replication, knobheterochromatin being thelast chromo- somal DNAtocomplete replication. Nucleolus organizer het- erochromatin replicates during theperiod ofeuchromatin DNA replication. Theproportion oftheSperiod involved inasyn- chronous late replication isdirectly dependent ontheamount ofheterochromatin inthenucleus. Bothadditional knoband B-chromosome heterochromatin participate inthis effect.- Each additional Bchromosome produces anincrement inthepro- portion ofnuclei withasynchronous replication; innuclei with 12Bchromosomes, theproportion oftheSphase showing asynchronous late replication ismorethan 3times greater than that innuclei without Bchromosomes. Thedata areconsistent with thehypothesis that delayed DNAreplication ofknobhet- erochromatin isresponsible, insomegenotypes, fortheloss of chromosome segments andforB-chromosome nondisjunction inthesecond pollen grain mitosis. Delayed replication ofheterochromatin intheknobs ofmaize chromosomes andinthesupernumerary Bchromosomes has beenimplicated asthecause ofboth theloss ofchromosome segments andthenondisjunction ofBchromosomes atthe second pollen grain mitosis (1, 2). Genetic studies using trans- locations between thenormal chromosomes ofthecomplement andBchromosomes haveshownthat tworegions oftheB chromosome areinvolved innondisjunction (3-5). Theproximal region near thecentromere, which contains aheterochromatic knob, andthedistal tipmustboth bepresent inamicrospore fornondisjunction tooccur. WhenaBchromosome isdissected into twopieces byatranslocation, only thetranslocated chro- mosomewiththeBcentromere undergoes nondisjunction. Rhoades andDempsey (1, 2)suggested that delayed replication oftheheterochromatic knobadjacent tothecentromere ofthe Bchromosome prevents theseparation ofthetwochromatids atanaphase andresults innondisjunction. Theother phenom- enon, theloss ofchromosome segments, depends uponanin- teraction ofBchromosomes andheterochromatic knobs onthe Achromosomes. Insomegenetic backgrounds, thepresence of twoormoreBchromosomes results inchromosome breakage andloss ofgenetic markers. Breakage isrestricted tochromo- somearmsthat bear aknobandoccurs intheregion between theknobandthecentromere, resulting intheloss oftheknob andthechromosome segments distal tothebreakpoint. Rhoades andDempsey (2) proposed that failure ofknobreplication in thesecond microspore mitosis causes theformation ofachro- mosomebridge atanaphase andthat subsequent breakage of thebridge results inloss ofchromosome segments containing theknob. Therationale forthis hypothesis isthat heterochro- matin inmanyorganisms isknowntobelate replicating (6). In Thepublication costs ofthis article weredefrayed inpart bypage charge payment. This article musttherefore behereby marked "ad- vertisement" inaccordance with18U.S.C.§1734 solely toindicate this fact.