Rick, Charles M. : Charles M. Rick, Professor, Vegetable Crops, U.C., Davis.

is that they are sometimeslinkedto important have been found . Thus we can identify resis - economictraits . Likegenesfor any character , tant plants at a much earlier stage , distinguish those coding for isozymes occupy specific resistant from susceptible plants more reli - loci on the chromosomes and are therefore ably because testing against the parasite is subject to the linkage relationship . That is , often beset with technical problems , and dis - the closer two genes are to each other on the tinguish between homozygous ( Mi / Mi ) and chromosome , the stronger is their tendency ( Mi / + ) plants - a task that heterozygous to be inherited together . Linkages between otherwise requires progeny tests for nema - isozyme markers and important economic tode susceptibility . lsozymes in traits are being exploited in tomatoes , since Linkages with isozyme loci can also assist the lack of effect in breeding programs dealing with quantita - of isozymes on appearance plant breeding tive characteristics ( such as earliness or and performance renders them superior to the customary marker genes used for this yield ) . In progenies segregating simultane - Charles M . Rick ously for such metric traits and isozymes , we purpose . can estimate the minimum number of genes One example is a remarkably tight linkage determiningthe metrictraits , localizespecific ( Aps - I ) between an acid phosphatase locus Isozymes are multiple molecular forms of metric genes by tests with two or more linked and the gene for resistance to root - knot ne - an enzyme derived from a tissue of an organ - ( Mi ) in tomato . The Mi gene was isozyme markers , and detect the presence of matodes ism . They are usually separated when an elec - discovered in a wild tomato relative , Lyco - plus or minus modifier genes . There is evi - tric charge causes their migration through a dence that selection for linked isozyme persicon peruvianum , and was invariably as - gel , and they are visualized when the gel is markers might be more reliable in improving sociated with a clearly defined variant acid placed in a solution of the proper substrate a metric character than selectionfor the char - L . peru - phosphatase band . Hybrids between for the enzyme and the end product of the re - vianum and cultivated tomato , L . esculen - acter itself . Another application is the more action is stained . This procedure results in tum , have both parental bands and an inter - rapid elimination of undesired wild - parent discrete bands , whose positions in the gel are mediate â?? hybrid â?쳌 band in the mid - position . traits that can result from intense selection determined by the charge and molecular against the wild - type form All three possible genotypes can be detected of the isozymes weight of the isozymes . The preparation is when genes are introduced into a cultivated of stems , roots , or from simple preparations usually a simple extract , sometimes just the leaves of plants of any stage . To date , almost a wild relative . plant by crossing with unmodified juice obtained by crushing the no exceptions to complete association be - Charles M . Rick , Professor , Vegerable Crops , plant sample . U.C . , Davis . Mi tween the slow acid phosphatase band and Since enzymes are coded by genes , the method has the great advantage of revealing gene activity much more directlythan do the end products of the familiar structural and physiological characters . In hybrid pro - genies , the segregation of isozymes , as detected by by the band patterns , determines which bands are variants corresponding to a particular geneticlocus ( that is , a position on a chromosome ) and which belongs to other loci . Isozymes permit unequivocal identifica - tion of nearly all genotypes . In contrast to the segregation of classical structural / physiolog - ical characters ( tall vs . dwarf , early vs . late ) , for which dominance and gene interaction seldompermit direct genotypedetection , this of iso - can be accomplished by examination zyme banding patterns , whether for alleles at one locus or many . The method is rapid , economical , and accurate , and the analysis ApS - 1 : + I1 + I1 + I + + I1 111 + / I + I + can be made for nearly all loci in earlygrowth Mi : + lMi + lMi + I + + lMi MilMi MilMi + lMi + I + stages , sometimes even in the seed itself . Nematode Resis . Resis . Resis . Resis . Susc . Resis . Susc . Resis . Isozyme analysis has been used with a number of cultivated plants to determine the degree of inbreeding , resolve problems in tax - Segregation of acid phosphatase isozymes ( APS - 7 ) as seen in onomy , study chromosome pairing in poly - starch gel banding patterns of eight plants in F2segregation . ploids , and identify cultivars . Isozymes can Below that is corresponding + / Mi segregation and susceptibility so that it could be in - be bred into a cultivar or resistance to nematodes . Resistant class includes both MiIMI ( homozygous or true - breeding resistant ) and + / Mi ( heterozygous controvertiblyidentifiedfor legal purposes - or segregating resistant ) . a possible form of control for patented vari - eties . One of the most useful aspects of isozymes 28 CALIFORNIA AGRICULTURE , AUGUST 1982