When generating kelp hybrid crosses in vitro, small blades are frequently s
een in single-sex cultures of microscopic gametophytes as well as in putati
ve hybrid crosses. We demonstrate a polymerase chain reaction (PCR)-based m
ethod that quickly determines the presence/absence of possible parental gen
omes in the observed blades. ITSI nrDNA fragments that varied in length in
a species-specific way were amplified. Amplification products from possible
hybrid blades, apogamic blades and parental gametophytes were compared on
agarose gels. Using these methods, we were able to identify a number of apo
gamic blades (i.e. formed directly hom gametophytic cells) as well as a tru
e hybrid sporophyte blade in crosses between Alaria marginata (Alariaceae)
and Lessoniopsis littoralis (Lessoniaceae), members of two separate familie
s of the Laminariales. To our knowledge this is the first genotypic confirm
ation of a hybrid kelp.