HYB:Chromosome compatibilities among Eupogons

In the extended thread about SWERTI the question arose about chromosome
distinctions between *pallida* and *variegata.*  It could be helpful to
broaden that issue a bit.

The karyotypes of *pallida* and *variegata* are published side by side in an
illustration in *Garden Irises.*   In the photos there appears to be a
difference in chromosome length between the karyotypes, possibly a laboratory
artifact, but it is remarkable how very much alike they are.  From the number
one long metacentric from both karyotypes right down through the sequences of
acrocentrics with and without satellites is almost identical between the
species.

I recall seeing a karyotype of *kashmiriana,* although I have no idea what
clone or form, and it too was nearly identical to the above.

These similarities suggest a relatively slight evolutionary divergence of
these various species from their common ancestors, despite the phenotypic
differences.  The long standing hybrid swarms of various origins also suggest
that more than minor compatibilities exist among the extant species.

In order for any significant fertility to occur in the diploid hybrids there
needs to be a high degree of homology in the sequences of base pairs in the
DNA along large proportions of the pairing chromosomes.  Otherwise meiosis
cannot proceed.  Homology to a high degree also facilitates cross-overs.

In the hybrid swarms the intergrading of types suggest that the genomes
combine seamlessly into a genetic pool within which segments flow back and
forth among the matching chomosomes almost without hindrance. *I. pallida*,
and *variegata* may not be the only species involved in these swarms, but
presumably dominate those from which the non-species Amoena, Plicata,
Squalens, et al. were selected.  Few of these lack abundant fertility.

Tom Silvers' CLOWN PANTS, a much wider hybrid between a *variegata* clone and
*suaveolens* Mellita is also fertile.  So are his *pallida* Kupari X
*suaveolens* Rubromarginata seedlings, about half of which show the I(s)
dominant Progenitor-type amoena factor, apparently present in several Balkan
dwarfs.  Peyrard's *timofejewii* X *variegata* hybrid is also fertile--all of
these at the diploid level.

There is also no question of fertility of the progeny from back-crosses from
PROGENITOR, itself presumably an amphidiploid, to Tall Beardeds.  Similarly,
the (Sharkskin x *balkana* [tet]) hybrid from Gerald Darby has produced
numerous offspring in similar backcrosses, yet in all these there would be an
"odd man out" set of chromosomes markedly reducing fertility were not the
species' various genomes homologous to a substantial degree.

None of these crosses noted are from combinations that have as high a degree
of affinity as that which exists between *variegata* and *pallida.*   Crosses
from either species with Asiatic tetraploids that step up to the tetraploid
level are also generally quite fertile in successive combinations among them
or from them to other tetraploids.

Similarly, none of the *aphylla* hybrids, either those ancestral to early
varieties such as DYMIA and its child BLACK FOREST, or to more recent
developments such as those coming from several hybridizers on the west coast,
show any conspicuous lack of fertility.

The degree of affinity should not be surprising.  Recent genomic studies
involving specific enzyme systems show nearly identical base-pair sequences
across not only the species within a genus, but between widely divergent
genera as well.  An example is in the F3',5' H enzyme that is responsible for
mediating the swap from -H to -OH radicals at two positions on the core
anthocyanin molecule, diverting the synthesis from its progression toward
Pelargonidin over to that of Delphinidin.

Randolph had suggested decades ago that we might progress toward red pigments
in the anthocyanins should mutations occur dropping one or two of the
hydroxyls from the B ring of the anthocyanin pigment molecule.  Since his day
progress in molecular genetics has been so great that not only is the
mechanism identified (and patented) that could accomplish what he envisioned,
the exact sequences of the 1755 base pairs in the DNA is now known and
published--from several different species--responsible for the enzyme that
tacks on the offending hydroxyls.  The enzymes are nearly identical even
between species as widely separated as monocots and dicots.

If so close an affinity between widely separated genera can be shown by just
this one enzyme, the high degree of affinity between species within specific
sections or subsections of the genus Iris should not be surprising.  Indeed,
it would be expected.

Neil Mogensen

---------------------------------------------------------------------
To sign-off this list, send email to majordomo@hort.net with the
message text UNSUBSCRIBE IRIS
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index