Although achenes obtained from interspecific hybridization germinated,
they did not develop well and died at the hypocotyl phase,
which is the post-zygotic barrier to hybridization. Marta et al.
(2004) reported that post-zygotic barriers were observed in diploid
and octoploid crosses and their reciprocals, and the achenes from
interspecific hybridization of diploid and octoploid species showed
that the development of embryos and endosperm was poor, as
some serial microtome sections did not show these tissues. Similarly,
achenes from interspecific hybridization of octoploid and
diploid species grew abnormally, not only in terms of pollen grain
germination and tube growth, but in embryo and endosperm development
as well.
All progenies from crosses between ‘UC-01’ and ‘Hawaii-
30’ were diploid with the exception of one tetraploid plant.
Although pentaploids were thought to be originated from reduced
gametes (x + 4x) from crosses between diploids and octoploids, the
tetraploid originated from unreduced gametes (4x) from crosses
between diploid species. Yarnell (1931) reported that the 8x
progeny from a cross between F. vesca and F. virginiana seemed to
be derived from a combination of the doubled unreduced gamete
(4x) of F. vesca and the reduced gamete (4x) of F. virginiana. Yanagi
et al. (2010) reported that the hexaploids (6x) probably originated
from the embryo composed of the unreduced F. vesca gamete (2x)
and the reduced F.
×
ananassa gamete (4x). Because tetraploidy is
related to the ratio of unreduced gametes in the 2n gametes, the
frequency of occurrence of tetraploids depends on how many unreduced
gametes will be produced between the male and female
species used as parents. Actually, measurements of pollen size in
the present study (Table 4) showed that ‘UC-01’ and ‘Hawaii-30’