Interspecies Variation and Recombination of Mitochondrial DNA in the Aspergillus nidulans Species Group and the Selection of Species-specific Sequences by Nuclear Background

Abstract

The filamentous ascomycete Aspergillus nidulans has a 33-kb circular mitochondrial genome, certain regions of which have now been sequenced (Küntzel et al.; Davies et al.; both this volume). Interspecies variation in genome size has been observed between closely related members of the A. nidulans species group (Earl et al. 1981) (Fig. 1) and results from inserts into, or deletions from, what are basically similar restriction endonuclease maps (eight enzymes) in which most sites are conserved. By analogy with Saccharomyces (Borst and Grivell 1981), it seems possible that these variable regions result from addition or loss of introns, and sequence comparison of the regions in question should settle this. Insert 4 of A. nidulans var. echinulatus has been accurately located by heteroduplex analysis (Earl et al. 1981) and seems to lie within the apocytochrome-b gene (Davies et al., this volume).

Despite the overall similarity of the mitochondrial genomes shown in Figure 1, the three strains can be regarded as closely related but distinct species on the grounds of their behavior during sexual crosses. We have investigated the tolerance/requirement of the variable regions by the different nuclear backgrounds. This can be carried out only by protoplast fusion, since the species are heterokaryon-incompatible.

TRANSFER OF EXTRANUCLEAR MARKERS FROM A. NIDULANS TO A. NIDULANS VAR. ECHINULATUS
By selection for an extranuclear marker such as oli (Rowlands and Turner 1975), attempts were made to transfer the mitochondrial genome of nidulans to the echinulatus nuclear background (Fig. 2a). Analysis of the mtDNA from 15 hybrids, selected...