Sexuality, Incompatibility, and Intersterility in the Biology of the Sistotrema Brinkmannii Aggregate

SUMMARY

Ninety-six representatives of the Sistotrema brinkmannii aggregate have been assembled from North America, Europe, Australia, and the Middle East. The existence of primary homothallic, bipolar heterothallic, and tetrapolar heterothallic patterns of sexuality within the aggregate has been confirmed. The geographical and substratal ranges of the isolates indicate that homothallic forms are not restricted in either respect, nor are homothallic forms geographically or ecologically isolated from bipolar heterothallic forms. Five intersterility groups were observed among 74 bipolar isolates. Geographical or substratal barriers do not account for the intersterility of the bipolar groups; however, intersterility patterns coupled with substratal specificity may indicate the pathways of evolutionary divergence for some groups. Similarly, two intersterile groups were observed among six independently isolated tetrapolar cultures. Neither geographical nor substratal barriers appear to be involved in the intersterility of tetrapolar forms. In the laboratory, the fruiting of homo- karyons of heterothallic forms (both bipolar and tetrapolar) was common; however, none of the sporulating, heterothallic, field isolates was identified as a homokaryon. Absolute intersterility between bipolar and tetrapolar strains has been demonstrated in 3,066 unique pairings.