a little chromosome essay

I thought it might be helpful to the (few) people who were interested
in the discussion of arilbred chromosomes to give a short, simple explanation
of how chromosome makeup affects fertility.

Before the purists have me cuffed and taken away, I'll note that what
follows is a simplified generalization; I've left out the exceptions and
subtleties (of which there are many). So this is directed at people who
want to learn the basics now, so they can pick up the details afterwards.

Tall bearded, dwarf bearded, and aril irises can be crossed with each other
to produce all sorts of hybrids. Some types of hybrids tend to be fertile,
while others tend to be sterile. For the most part, fertility depends on
what combinations of chromosome sets the iris has.

Tall bearded irises have four sets of 12 chromosomes, for a total of 48.
If we represent a set of TB chromosomes by the letter T, then we can
write this as TTTT.

Aril irises have sets of 10 or 11 chromosomes. Some arils are diploids
with two sets, some are tetraploids with four sets. Using the letter A
to stand for a set of aril chromosomes, we can write AA (diploid aril)
and AAAA (tetraploid aril).

Iris pumila, a miniature dwarf bearded species from Europe, has four
sets of 8 chromosomes each. This can be written PPPP.

***
The fertility rule: If you can divide an iris's chromosome sets in
half symmetrically, it is likely to be fertile. If you can't, it is
likely to be sterile.
***

Examples: TTTT can be divided into TT and TT, so tall beardeds are fertile.
AA can be divided in A and A, so diploid arils are fertile. In fact, each
of the examples above is fertile.

But let's look at the hybrids. If I cross a TB with Iris pumila, the
result will be a standard dwarf, with half of its chromosomes from the
TB and half from pumila: TTPP. This combination is fertile, because
I can divide these chromosomes in half symmetrically: TP and TP.

But now if I cross an SDB with a TB, the offspring inherits TP from
the SDB parent and TT from the TB parent, giving it TTTP. This combination
is not fertile (generally), because TTTP cannot be divided symmetrically.
The only possible division is TT and TP, which is not symmetrical.

A little thought will show that there are only a few different types of
hybrids involving arils, TBs, and Iris pumila that will be fertile. These
groups are often called "fertile families":

Diploids: TT (miniature tall bearded), AA (diploid arils), PP (a few obscure
dwarf species, such as Iris attica).

Tetraploids: TTTT (TBs and BBs), AAAA (tetraploid arils), PPPP (Iris pumila),
TTAA (OGB arilbreds of the C.G.White type), TTPP (SDBs),
and AAPP (aril-pumila hybrids, of which only a few have been produced).

Everything else is sterile, to first approximation. (Yes, there are
exceptions, for which hybridizers are eternally thankful.)

This shows one of the reasons tetraploids are so important in breeding.
If you cross a diploid TB with a diploid aril (this kind of thing was
done a hundred years ago to make the first arilbreds), you get TA, which
is not fertile because it won't divide in half symmetrically. But if
you cross a tetraploid TB with a tetraploid aril, you get TTAA, which
works fine!

Here's a quick run-down of the most usual chromosome makeup of the
different classes of bearded and aril irises. Note that most classes
are not defined in terms of chromosomes, so there is no guarantee; this
list is a generalization.

TB, BB:      TTTT
MTB:         TT (a few TTTT these days)
IB:          TTTP
SDB:         TTPP
MDB:         TPPP (plus quite a few TTPP and PPPP)

pure aril:   AA (a few AAAA)
OGB+:        AAT (a few AAAT)
OGB:         AATT
OGB- (tall): ATTT
OGB- (arilbred median): ATTP


===============================================================

Tom Tadfor Little         tlittle@lanl.gov  -or-  telp@Rt66.com
technical writer/editor   Los Alamos National Laboratory
---------------------------------------------------------------
Telperion Productions     http://www.rt66.com/~telp/
===============================================================
Other Mailing lists | Author Index | Date Index | Subject Index | Thread Index