HYB: Wide cross (long sorry)

Iris-talkers,  On another list Karl King often posts about genetic and
hybridization, this post is particularly interesting. If this sort of
discussion is your cup o' tea you may want to check out his website
http://www.geocities.com/RainForest/1978/  where he has more articles -
particularly on hybridizing, roses and amaryllis

Chad Schroter

Date: Thu, 24 Oct 2002 15:35:34 -0700 From: Karl King 
Subject: Wide hybrids in Iris
 Systematists inform us that "crossability" is no longer an important
criterion for their classifications, which is of no concern to us. But then
they go on to assure us that their classifications probably define the
limits of crossability, which makes no sense to me.
 Nevertheless, plant breeders -- experts and amateurs -- go right on
attempting crosses that one systematist or another would certainly deem
impossible 
Many "impossible" hybrids have been raised, and there have probably been
even more unreported failures. We may fairly ask why crosses fail, and why
sometimes they succeed. There is no simple answer, because there are many
ways for a cross to fail. And when a cross succeeds, there are also a
variety of reasons why it might be sterile -- or, occasionally fertile. 
In flowering plants, the endosperm is a hybrid tissue, as is the embryo
itself. This fact gives a difficult cross two opportunities to fail: the
hybrid embryo may develop while the endosperm fails, and the endosperm may
succeed while the embryo aborts. If we know which failure prevents us from
raising a desired hybrid, we can take steps to preserve it. Embryo rescue
techniques for some plants have been developed, which gets us past the
problem of endosperm failure. And endosperm tissue may be extracted,
cultured and induced to regenerate plantlets -- this time allowing us to
raise hybrids after the embryo has withered. 
Artificial techniques make the business easier, but now and again plants
manage to produce a few viable seeds against the odds. These are all we find
in the older reports, before the days of artificial techniques. 
Crosses of flowering plants also may fail before fertilization occurs.
Pollen may be rejected at the stigma, destroyed in the style, or it may be
incapable of growing the length of the style in order to reach the ovules.
This sometimes occurs where the parents differ greatly in style length. In
some cases it is possible to avoid this difficulty by shortening the style
and pollinating the stump, or grafting on a piece of stigma from the
paternal species (from a clone that is cross-compatible with the pollen
parent). 
Finally (?), hybrids may be sterile because, for a variety of reasons, the
fail to produce viable pollen. Even this is not a simple problem. "Genetic
incompatibility" is a catch-all explanation that does not explain all cases.
One hybrid may be perfectly sterile, while another from the same parents may
produce a small percentage of viable pollen grains, though the hybrids share
most of the same genes. Of course, we need only one viable pollen grain to
continue our experiment to the next generation, so "almost sterile" may be
good enough. 
Lenz (1955) discussed some difficult Iris hybrids and found differences
among the sections of the genus in how they responded to artificial embryo
culture. He "Excised embryos of I. hoogiana Dykes, I. korolkowii Regel, and
I. stolonifera Maxim. When grown on nutrient agar under conditions favorable
for the growth of tall-bearded iris embryos usually failed to germinate at
all and instead became enlarged and malformed." Werckmeister had similar
results where the seed parent of an interspecific hybrid was of the
Oncocyclus and Hexapogon (Regelia of Dykes) sections and the pollen parent
was of any other section. 
After some experiments with stratification (chilling), Lenz found that the
problem with the Hexapogons and their hybrids was embryo dormancy that could
be broken by chilling for ten weeks at 38F. This "was found to produce up to
93% germination of the embryos when they were later placed in the
germinator." 
In a separate report, Lenz (1954) discussed endosperm failure in Iris
hybrids. "In the iris breeding program being carried on here [Rancho Santa
Ana Botanic Garden], it has been our experience that capsules produced in
many intersectional crosses may contain ovules of varying degrees of
normality as regards size and appearance. However, upon drying, the seeds
collapse and in most instances would be classified as aborted. Nevertheless,
occasionally one of the larger seeds will germinate and produce a viable
seedling, the embryo in this case apparently having passed the threshold
limit of germinability in its development." 
A TB was pollinated by I. tectorum. The capsules were harvested when about
half mature and the seeds dissected. "It was discovered that in nearly all
of the seeds the central cavity which at this stage of development should
contain a rather firm white endosperm, contained either a thin colorless
liquid, or was devoid of any substance. Upon probing it was possible to find
in almost every ovule a rather normal looking though flaccid embryo." 
A similar condition of endosperm was found in a cross of I. missouriensis x
I. Munzii (of subsections longipetalae and californicae respectively).
Finally, in 1956 Lenz reported on two successful hybrids that had been
raised between a diploid bearded iris and I. tectorum, and a second from I.
douglasiana x I. sibirica 'Caesar's Brother'. 
"This is probably the third record of a hybrid being produced between a
bearded and a non-bearded iris. Dykes in 1910 reported a hybrid which he had
obtained from a cross between the dwarf form of I. pallida that Sir Michael
Foster had obtained from Monte Loppio (later called I. cengialti var.
loppio) and I. tectorum." It was named 'Lop-tec'. 
The second cross, between I. tectorum and the bearded 'Edina', was recorded
in 1928 by F. Denis, a French horticulturist. This 'Pal-tec' is pictured in
the article. 
"Hybrids between such widely separated taxa are usually sterile and since
1928 when 'Pal-tec' was registered to the present time no seedlings have
been obtained from this variety. Pollen from the 'Ariel' x tectorum cross
was treated with a lacto-phenol-aniline blue solution and when examined
later a number of pollen grains were found to be stained. This was
surprising considering the extreme differences between the parental species.
Plans were made to attempt backcrosses with the parental plants, but
unfortunately this hybrid and its sibling succumbed during the extreme heat
wave in southern California in September, 1955." 
Another natural disaster! The one time I saw a genuine fireball in the sky,
I wasn't carrying my camera. And if Bigfoot ever crosses my path, I will
surely find that I used my last frame photographing a wild flower. But at
least we know that a hybrid of a TB iris with I. tectorum *may* be at least
partially fertile, which is reason enough to continue making similar
crosses.
It may be coincidence, but I think it's worth noting that the sterile
hybrids were raised without artificial culture, while the one slightly
fertile specimen was preserved only through embryo rescue.
I recall a possibly analogous series of results obtained in a number of
interspecific Paeonia crosses. Some pairs of species crossed readily, but
gave sterile or weakly fertile hybrids; other pairs crossed only with
difficulty, but the hybrids were quite fertile. 
Off hand, I cannot think of any mechanism or pair of mechanisms that
*should* work reciprocally, leading EITHER to embryo failure OR to sterility
in the mature plant. Even so, I feel obliged to note the coincidence. 
Karl King
Lenz, L W (1954) The endosperm as a barrier to intersectional hybridization
in Iris.El Aliso 3 (1): 57-58 
Lenz, L W (1955) Studies in Iris embryo culture. El Aliso 3 (2): 173-182 
Lenz, L W (1956) Two new and unusual Iris hybrids. El Aliso 3 (3): 345-349






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