Seventy-two hybrids of P. aquatica were made among 13 diverse Mediterranean ecotypes and cv. Australian,
and five indicators of hybrid abnormality were measured in the first two generations. The proportions of
stainable, apparently normal pollen grains formed by F, hybrids varied between 5 and 98% and their spikelet
fertility ranged from 19 to 77%. Of the 43 hybrids that were derived from pairs of self-incompatible parents,
13 were highly self-compatible. Crosses between lines from different groups of parents produced F2 progeny
containing up to 17% of lethal seedlings and up to 39% of male sterile plants. In the cross between cv.
Australian and the Moroccan ecotype, CPI 19331: the frequency ofzebra-striped lethal seedlings was as high
as 33% in F2 progenies, but only 0 or 1% in back-crosses to either parent. Hence the zebra-striped phenotype
was not caused by a mutation existing in the parents but rather by deletions or duplications generated during
meiosis in the F1 hybrids.
Many meiotic irregularities were observed in the pollen mother cells of the F1 and F2 hybrids between cv.
Australian and CPI 19331. These included small loops, acentric fragments, univalents and multivalents at
diakinesis, and bridges at anaphase I, indicating that the genomes of these two lines differed by several
inversions and interchanges. These structural differences would lead to a range of duplications and
deficiencies in the gametes. and hence could account for each of the five kinds of hybrid abnormality
observed in the F1 and F2 generations. One aneuploid F2 plant with 25 chromosomes was found.
A dihaploid plant in cv. Australian had an average of 4.3 bivalents per pollen mother cell, whereas
virtually all chromosomes in the tetraploid parental lines paired as bivalents. Thus, P. aquatica is a
segmental allotetraploid with a system which prevents homoeologous pairing in tetraploids but not in
dihaploids.
The partial barriers to hybridization between P. aquatica lines are not closely related to their geographic
origins or varietal classifications. These barriers may hinder but have not prevented the recombination of
parental traits during the development of improved cultivars.
Chromosomal differentiation of schistosomes: what is the message?
