An experimental study of the reproductive biology and hybridization in the European and North American species of Equisetum

Sporophytc production was investigated in the eleven North American and European species of Equisetum by means of fertilization experiments on both intact and subcultured gametophytes. Comparison of intra- and inter-gametophytic selfing with intergametophytic crossing failed to reveal any evidence of recessive sporophytic lethals, leaky lethals or deleterious recessive genes. Fertilization in Equisetum depends solely on the availability of ripe archegonia. Despite the absence of detectable genetic load in Equisetum , intergametophytic mating is ensured by gametophyte dioecism in the wild. All populations tested were freely interfcrtile indicating a high degree oi genetic con-specihcity, even between North America and Europe. Low sporophyte frequencies in European E. scirpoides are attributed to defective spermatozoids. Polyembryony occurs. Cessation in gametophyte development is mediated by allelopathic compounds liberated from the young sporophytes.
Reciprocal crosses resulted in the synthesis of 12 hybrids (six in subgenus Equisetum and six in subgenus Hippochaete plus abortive zygotes in the cross E. ramosissimum female x E. scirpoides male), 11 of which had previously been reported in nature. No intersubgeneric hybrids were produced, indicating that both subgenera are natural groups only distantly related to one another. The putative wild combinations E. pratense x E. atvense and E. variegatum x E. scirpoides were not synthesized, but a new hybrid from the allopatric E. ramosissimum and E. laevigatum arose. Reciprocal crosses within the subgenus Hippochaete usually yielded equal frequencies of hybrids, but in the subgenus Equisetum the majority of successful crosses were unidirectional. All taxa oi Hippochaete are linked by hybrids whereas in subgenus Equisetum, E. pratense and E. sylvatkum form an isolated pair, Hybridization studies, and evidence from gametophyte morphology support the traditional sectional classification of Equisetum.