In the last few years many genes of several Aspergillus species have been cloned and sequenced. For many of these genes mutant alleles and genetic linkage data are also available. However, for those genes for which no mutant alleles have been isolated, genetic mapping was not possible. Here we report linkage mapping of the glyceraldehyde-3- phosphate dehydrogenase gene (gpdA) of A. nidulans for which no mutant alleles have been isolated. The method used is applicable to all other cloned genes.
Transformation in Aspergillus frequently occurs by homologous recombination between host and vector sequences. Although the frequency of homologous recombination does not need to be identical for different sequences, a vector containing sequences of the gene to be mapped will often integrate at the chromosomal locus of this gene. In this way, A. nidulans ArgB[pAN5-41B]15 was obtained (vector pAN5-41B contains the lacZ gene fused to the promoter region of the gpdA gene of A. nidulans ; Van Gorcom et al. 1986 Gene 48:211-217). Southern blot analysis has shown that this strain contains a single copy of a (functional) lacZ gene at the gpdA locus. Parasexual analysis of this strain with master strain MSE (A. nidulans FGSC A288) was carried out. Segregation of the lacZ marker (integrated at the gpdA locus) was analysed (Table I).
Table I. Linkage group assignment
I II III IV V VI VII VIII
FGSC A288 yA2 wA3 galA1 pyroA4 facA303 sB3 nicB8 riboB2
gpdA:lacZ(a) 62/64 2/139 47/121 72/135 63/139 ND 61/133 59/138
* 1% 39% 53% 45% ND 45% 43%
ArgB[pAN5-41B]15 biA1 - argB2 methG2 - - - -
gpdA:lacZ 63/138 - ** 66/138 - - - -
46% - - 48% - - - -
a - the number and % of recombinants is given; two independent diploids were
analysed. The markers located at both arms of chromosome IV (pyroA4 and
methG2) gave 25/133 (19%) recombinants in this experiment.
* - wA3 is epistatic to yA2
** - only ArgB+ segregants were analysed
From the results in Table I we conclude that the lacZ gene is significantly linked to wA3 at chromosome II: thus, gpdA is located on chromosome II. Using suitable linkage group II strains the location of gpdA on chromosome II can be mapped similarly, although the presence of duplicated sequences in ArgB[pAN5-41B]15 could disturb normal segregation in sexual crosses.