Genetic characterization of somatic recombination in Trichoderma pseudokoningii

Abstract

Crossing experiments via hyphal anastomosis between two strains contrasting for auxotrophic markers of Trichoderma pseudokoningii were conducted to characterize the somatic recombination process in this specie. Four crossings were made and a total of 1052 colonies obtained from conidial suspensions of the heterokaryotic colonies were analyzed. Sixty-eight recombinant colonies, from four growing generations, were analyzed for the auxotrophic markers. Of the 68 colonies analyzed, 58 were stable after four generations and the remainders were unstable, reverting to one of the parentals. Most of the recombinant colonies were unstable through subculture and after four growing generations they showed the leu ino met markers (auxotrophic for leucin, inositol and metionin respectively). The unstable recombinant colonies showed irregular growing borders, sparse sporulation and frequent sector formation. The results suggest the occurrence of recombination mechanisms in the heterokaryon (somatic recombination), different from those described for the parasexual cycle or parameiosis. Therefore, we proposed the ocurrence of nuclei degradation from one parental (non prevalent parental) in the heterokaryon and that the resulting chromosomal fragments may be incorporated into whole nuclei of the another parental (prevalent parental). However the parameiosis as originally described cannot be excluded.

Limited vegetative compatibility as a cause of somatic recombination in Trichoderma pseudokoningii

“…As described by Stasz & Harman (48), the same pattern of recombinant progeny strains is frequently obtained from heterokaryons in several crosses between T. pseudokoningii contrasting strains (3, 7). …”

“…A total of 1,052 colonies obtained from conidial suspensions of heterokaryotic colonies from four crosses were analyzed, and most were unstable through subculture and characterized by irregular colony borders, sparse sporulation and frequent sector formation. All recombinant colonies obtained were morphologically similar and had the same auxotroph markers of one of the parental strains (parent 12L) (7). Due to the instability through subculturing of the recombinant progeny and the similarity to only one of the two parental strains, the authors suggested that a mechanism of somatic recombination might be occurring, as proposed by Satsz & Harman (48).…”

“…Whithin the Trichoderma genus there are descriptions of obtention of recombinants directly from the heterokaryon between strains of several species, which were formed by means of hyphal anastomosis, protoplast fusion or nuclear transfers (3, 7, 23, 37, 38, 46, 47, 48, 49, 50). In several experiments with Trichoderma , the occurrence of characteristic patterns has been observed and would be related to some level of vegetative incompatibility or due to limited vegetative compatibility (5, 46, 48, 49).…”

“…There are some reports of the obtention of recombinant progeny of T. pseudokoningii directly from the heterokaryon, by hyphal anastomosis or protoplast fusion, in which almost all progeny strains showed the auxotrophic markers from only one of the parents (3, 7, 23). As described by Stasz & Harman (48), the same pattern of recombinant progeny strains is frequently obtained from heterokaryons in several crosses between T. pseudokoningii contrasting strains (3, 7).…”

“…In a previous work, we conducted crossing experiments via hyphal anastomosis between two T. pseudokoningii strains with contrasting auxotrophic markers, aiming at characterizing the somatic recombination in this species (7). A total of 1,052 colonies obtained from conidial suspensions of heterokaryotic colonies from four crosses were analyzed, and most were unstable through subculture and characterized by irregular colony borders, sparse sporulation and frequent sector formation.…”