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Date: Mon, 28 Jul 2003 10:49:08 -0700 (PDT) 

Partial Hybrids in Practice
Karl King

When people imagine they are defending orthodoxy, they are not necessarily able to think clearly about their arguments, or about the evidence they are ignoring. Cases are dismissed as anecdotes. Hard scientific evidence only demonstrates that exceptions to the rules may exist — rarely.

When we ask how the "rules" became established, we are often met with a stony silence. That's the nice thing about being on the side of orthodoxy — no explanations are necessary.

I could go on citing cases, but that gets us nowhere. Instead I decided to search for some plausible mechanisms that might account for partial hybrids, and — if we are very lucky — some indications as to how we may provoke the phenomenon at will.

If anyone wants to do the same, let me warn you the path isn't easy. For one thing, I found very little of interest while searching for "partial hybrid". But as I thought about what may be happening, and thought back to something I learned in a Developmental Biology class many years ago (what a memory!), I finally started finding some leads.

For one thing I found the odd case of the jewel wasp, Nasonia vitripennis. In this species sex is determined by ploidy: females are diploid, males monoploid. Some males possess a small B chromosome that has no obvious effect. Since the males are monoploid, all their chromosomes — including the B — pass into each sperm cell. There is nothing really remarkable about any of this, but something strange happens when the sperm nucleus fuses with that of the ovum. Somehow, the B chromosome induces its brother chromosomes to condense and disintegrate. Thus all the progeny from the mating turn out to be males — just as though the female laid eggs without mating.

This odd case teaches us two important facts: the behavior or expression of a hereditary element can change abruptly when it is shocked by a change in its immediate cytoplasmic environment. In addition we see very clearly that chromosomes from the sperm nucleus are distinguishable from those native to the ovum — at least at first. If not, the B chromosome could not exert its lethal force (whatever it is) selectively on chromosomes received from the male parent.

I have another item on the tendency of chromosomes to remain "huddled" — those from the male parent in one part of the cell, those from the female in another — in the fertilized ovum and for a few divisions afterwards. Eventually this tendency fades, but by then — we may suspect — the embryo could be on the road to "partialism" (to coin a word).

The next phenomenon of note is asymmetric mitosis. Often cells divide so that the daughter cells are virtually identical. But for special occasions the cell becomes polarized, and various "destiny substances" are distributed towards the poles. The cell then divides across the equator giving two very distinct daughter cells with different destinies.

Sexual meiosis involves asymmetric divisions. In male germinal tissues half the cell is prepared for it future role as mother (father?) of pollen or sperm nuclei. The other half is programmed for elimination. Likewise, production of female gametes involves a sacrificial line and another that yields ova.

Various cytoplasmic components may be selectively retained or excluded in the polarization, and differently in male gametes than in female. Some plants routinely pass chloroplasts through their pollen, which means that chloroplasts are moved to the appropriate pole. In other plants unpaired chromosomes are differentially distributed. For example, species of the Caninae section of the genus Rosa are all polyploids (4x, 5x or 6x), but only two sets pair off so that they can be distributed equally in pollen and in ova. The unpaired chromosomes are retained in the ova, but always eliminated from the pollen. The opposite situation is seen in some hexaploid wheat carrying an unpaired rye chromosome. In one case, the great majority of pollen nuclei carried the chromosome, but only a minority of ova did. The difference between roses and the wheat-rye aneuploid is that the roses maintain the behavior as part of an ancient system. The wheat-rye strain involved a foreign chromosome that was carried along differentially in the migration of destiny substances.

The next major instance of asymmetric mitosis occurs with the first division of the fertilized ovum of a plant. It should be remembered that most of the destiny substances — whatever they happen to be — are of maternal origin. These can force some genes to turn off, and others to turn on — but generally in the direction of the maternal pattern of regulation. I could cite some cases of an apparent maternal influence, sometimes modified by environment, that affects the development of a plant but which is not specifically (i.e., genetically) inherited. Maybe another time.

The "bottom" half of the fertilized ovum will become the founding cell of the "suspensor", while the top goes on to produce the embryo. The bottom cell then divides asymmetrically again, but we can ignore it because it won't contribute to the future plant. The top cell divides symmetrically, then these two divide asymmetrically. The two bottom cells will form the hypocotyl, and the top two go on to form the cotyledons and shoot meristem.

This diagram gave me a little jolt. Years ago — about the time I took the course in Developmental Biology — I planted some rose seeds in a terrarium. This is not the best way to grow roses, but it was my first time and I didn't know any better. Soon I had about a dozen seedlings. Soon after I had about a dozen seedlings damping off. I was disappointed, but decided to discard the poor things. But as I pulled them out I noticed something odd. Some of the seedlings appeared to have been attacked only on the hypocotyl — the cotyledons and small shoots remaining plump. Other seedlings were attacked at the cotyledons and above, but the hypocotyls looked good. Some seedlings were altogether dead.

With nothing to lose, and having recently read an article about micrografting spruces, I found a rusty razor and started cutting. I bound good hypocotyls to good shoot portions with some thread then replanted them in the terrarium. I did not sterilize the soil, but I may have left the cover a bit more open to reduce humidity. I can't remember for sure. In any event, I managed to preserve two grafted plants with no further damping off. They eventually matured and bloomed, so it all ended happily.

The striking fact is that there was an apparent segregation of damping off resistance along the hypocotyl-cotyledon division — right where asymmetric divisions establish the distinction. I cannot claim that there was also genetic segregation. Maybe it was cytoplasmic. Something, anyway, appeared to have segregated with the asymmetric division.

Another, similar case was reported by Luther Burbank. He had pollinated a number of flowers of a Horticultural or "Wren's Egg" bean with pollen from a Lima. One pod gave four beans that looked like the ordinary Wren's Egg type. But as the seeds sprouted the cotyledons were seen to be partly of the Lima type. He wrote, "These parts were connected with serrated edges, which at last separated, allowing the lima bean part to drop away. Such separation, however, did not occur until the vines had made a foot or more of growth.     The cotyledons on each side were divided uniformly in every case."

Again, this appears to involve segregation at the hypocotyl/cotyledon boundary. The plants subsequently looked entirely like the Horticultural seed parent aside from being remarkably vigorous. The next generation reverted to the ordinary form, and no sign of the Lima was seen again.

But there I go citing examples again.

I did find a very interesting, and potentially useful item involving hypocotyls. This is the summary of a report published in Mol. Gen. Genet. 1995 Mar 20; 246(6): 657-62, in case anyone needs more information.

"During the establishment of an embryogenic cell line from a carrot hypocotyl explant, processes closely resembling meiotic divisions are seen. A microdensitometric analysis revealed that the amount of cellular DNA diminished in the majority of cells to the haploid level. However, the diploid level was re-established in a matter of a few days. The genetic consequences of this segregation were studied by analyzing restriction fragment length polymorphisms (RFLP) and randomly amplified polymorphic DNAs (RAPD). The results showed that the great majority of embryos regenerated from segregants and that different segregants had different genetic constitutions."

Robert Edwards, Ph.D., responsible for the first in vitro human fertilization, gave a speech on cloning. "It is not simple; don't think it is simple. It is extremely complex. Indeed, everything we do in vitro to a mammalian embryo causes it stress. Raised lactate uptake, aerobic glycolysis, lower oxidative metabolism. These are the authors. This is what happens to the metabolism of embryos simply placed in a culture medium without any treatment whatsoever. They're not being micromanipulated, they are just left in culture. Five times more HSP 70.1 synthesis. Five times increase in heat shock proteins. That is astonishing. I wonder what these heat shock proteins do. I know they're chaperone molecules, but I think they've got another function, which you don't have time to discuss today. Even liver protein expression was modified, according to Reid, and disaggregation of the embryos of micronafazer cause stress, a modified gene expression, according to Venet. So, you see, before we get the clone, we've battered that nucleus. It's changed its properties. It's switched on new types of factors, in addition to twins. Twins are just a straightforward separation of halving. That's a simple system. Cloning is a complex system. The results of cloning will be far wider than differences between twins because you've got fantastic biochemical processes taking place."

Though the details may differ, regenerating plantlets from a hypocotyl involves similar shocks — very different from taking root or shoot cutting. This may be bad for cloners, but could be very good news for hybridists.

Many hybrids are uniformly sterile, as we all know. We have no opportunities for discovering new combinations of traits in later generations. But if we could regenerate plantlets from hypocotyls of hybrid seedlings, perhaps we could find some segregation in the F1 generation. And because segregation requires that some chromosomes or parts of chromosomes are lost in the process, the regenerated plants would be partial hybrids — homozygous for some parental "genes" but not all. While this may not explain how partial hybrids form under "normal" conditions (although the introduction of foreign chromosomes could be shock enough), it may be a useful technique for provoking them. It is even possible that some of the explants will retain only a chromosome or two from one of the parents, and thus be at least partially fertile as well as partially hybrid. Crindonnas come to mind.


Komeda et al: Chromosome elimination in wheat hybrid (2007)

Straadt, Rasmussen: Solanum phureja (2003)

Other possible cases of partial hybridization are included under "Exceptional" Crosses.