Causes of the Sterility of first Crosses and of Hybrids

What are the probable causes of the sterility of first crosses and hybrids?

These two cases are fundamentally different.

The union of 2 pure species the male and female sexual elements are perfect. Whereas in hybrids they are imperfect.

Even in first crosses, the difficulty in effecting a union depends on several causes.

There might be a physical impossibility in the male element reaching the ovule.

An example is a plant having a pistil too long for the pollen-tubes to reach the ovarium.

When pollen of one species is placed on the stigma of a distantly allied species, the pollen-tubes protrude.

• But they do not penetrate the stigmatic surface.

The male element may reach the female element.

But it might be incapable of causing an embryo to be developed.

• This was seen in Thuret’s experiments on Fuci.

An embryo may be developed, and then perish at an early period.

Mr. Hewitt has hybridized gallinaceous birds.

He told me that the early death of the embryo is a very frequent cause of sterility in first crosses.

Hybrids are generally healthy and long-lived, as we see in the mule.

Hybrids, however, are differently circumstanced before and after birth.

A hybrid has only half of the mother’s nature and constitution.

It may experience unsuitable conditions that might kill it early.

The sexual elements of hybrids are imperfectly developed, causing their sterility.

When animals and plants are removed from their natural conditions, their reproductive systems are seriously affected.

This is the great bar to the domestication of animals.

Between the sterility thus superinduced and that of hybrids, there are many points of similarity.

In both cases, the sterility is:

• independent of general health
• accompanied by excess of size or great luxuriance.

In both cases:

• the sterility occurs in various degrees.
• the male element is the most liable to be affected; but sometimes the female more than the male.

• whole groups of animals and plants are rendered impotent by the same unnatural conditions produce sterile hybrids.

On the other hand, one species in a group might:

• resist great changes of conditions with unimpaired fertility
• produce unusually fertile hybrids

No one can tell whether:

• an animal will breed under confinement
• a plant will seed freely under culture
• any 2 species of a genus will produce sterile hybrids.

Lastly, when organic beings are placed for several generations under conditions not natural to them, they vary.

I think this is from their reproductive systems being specially affected, though in a lesser degree than when sterility ensues.

So it is with hybrids, for hybrids in successive generations are eminently liable to vary, as every experimentalist has observed.

The reproductive system is affected when:

• organic beings are placed under new and unnatural conditions
  • Here, the conditions of life are slightly disturbed
• hybrids are produced by the unnatural crossing of 2 species
  • Here, the organisation has been disturbed by 2 different structures and constitutions being blended into one

When hybrids are able to breed inter se, they transmit to their offspring from generation to generation the same compounded organisation.

• Hence, their sterility rarely diminishes.

We do not understand:

• the unequal fertility of hybrids produced from reciprocal crosses
• the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent.

In 2 cases, in some respects allied, sterility is the common result.

In the one case from the conditions of life having been disturbed, in the other case from the organisation having been disturbed by two organisations having been compounded into one.

Slight changes in the conditions of life are beneficial to all living things.

We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, &c., from one soil or climate to another, and back again.

During the convalescence of animals, great benefit is derived from almost any change in the habits of life.

A cross between very distinct individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring.

A certain amount of crossing is indispensable even with hermaphrodites.

Close interbreeding continued during several generations between the nearest relations, especially those kept under the same life conditions, always induces weakness and sterility in the progeny.

Hence, slight changes in life conditions benefit all organic beings.

Slight crosses are crosses between the males and females of the same species which have become slightly different.

• This gives vigour and fertility to the offspring.

Greater changes, or changes of a particular nature, often render organic beings sterile.

Greater crosses are crosses between males and females which are widely different, produce hybrids which are generally sterile in some degree.

This parallelism is not an accident or an illusion.

There is some essential distinction between species and varieties.

Varieties that differ from each other in external appearance but:

• cross with perfect facility
• yield perfectly fertile offspring

I fully admit that this is almost invariably the case.

If 2 reputed varieties are found sterile together, they are at once ranked by most naturalists as species.

For instance, the blue and red pimpernel, the primrose and cowslip are varieties to our best botanists.

But Gartner says they are not fertile when crossed.

• And so he ranks them as species.

If we thus argue in a circle, the fertility of all varieties produced under nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in doubt. For when it is stated, for instance, that

The German Spitz dog unites more easily than other dogs with foxes.

Certain South American indigenous domestic dogs do not readily cross with European dogs, the explanation which will occur to everyone, and probably the true one, is that these dogs have descended from several aboriginally distinct species.

Nevertheless, the perfect fertility of so many domestic varieties, differing widely from each other in appearance, for instance of the pigeon or of the cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed.

Several considerations render the fertility of domestic varieties less remarkable than at first appears.

It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties.

In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile; and if this be so, we surely should not expect to find sterility both appearing and disappearing under nearly the same conditions of life.

Lastly, and this seems to me by far the most important consideration, new races of animals and plants are produced under domestication by man’s methodical and unconscious power of selection, for his own use and pleasure: he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system.

He supplies his several varieties with the same food; treats them in nearly the same manner, and does not wish to alter their general habits of life.

Nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature’s own good; and thus she may, either directly, or more probably indirectly, through correlation, modify the reproductive system in the several descendants from any one species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed.

But it seems to me impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which I will briefly abstract.

The evidence is at least as good as that from which we believe in the sterility of a multitude of species. The evidence is, also, derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction.

Gartner kept in his garden:

• a dwarf maize with yellow seeds
• a tall variety with red seeds, growing near each other

These plants have separated sexes, they never naturally crossed.

He then fertilised thirteen flowersof the one with the pollen of the other; but only a single head produced any seed, and this one head produced only five grains. Manipulation in this case could not have been injurious, as the plants have separated sexes. No one, I believe, has suspected that these varieties of maize are distinct species; and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile; so that even Gartner did not venture to consider the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the maize has separated sexes, and he asserts that their mutual fertilisation is by so much the less easy as their differences are greater. How far these experiments may be trusted, I know not; but the forms experimentised on, are ranked by Sagaret, who mainly founds his classification by the test of infertility, as varieties.

The following case is far more remarkable, and seems at first quite incredible; but it is the result of an astonishing number of experiments made during many years on nine species of Verbascum, by so good an observer and so hostile a witness, as Gartner: namely, that yellow and white varieties of the same species of Verbascum when intercrossed produce less seed, than do either coloured varieties when fertilised with pollen from their own coloured flowers.

Moreover, he asserts that when yellow and white varieties of one species are crossed with yellow and white varieties of a distinct species, more seed is produced by the crosses between the same coloured flowers, than between those which are differently coloured.

Yet these varieties of Verbascum present no other difference besides the mere colour of the flower; and one variety can sometimes be raised from the seed of the other.

From observations which I have made on certain varieties of hollyhock, I am inclined to suspect that they present analogous facts.

Kolreuter has proved that one variety of the common tobacco is more fertile when crossed with a widely distinct species.

He experimentised on 5 varieties via reciprocal crosses.

He found their mongrel offspring perfectly fertile.

But one of these 5 varieties, when used either as father or mother, and crossed with the Nicotiana glutinosa, always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with N. glutinosa.

Hence the reproductive system of this one variety must have been modified.

It is difficult to ascertain the infertility of wild varieties

A supposed variety if infertile would be ranked as species.

from man selecting only external characters in the production of the most distinct domestic varieties, and from not wishing or being able to produce recondite and functional differences in the reproductive system; from these several considerations and facts, I do not think that the very general fertility of varieties can be proved to be of universal occurrence, or to form a fundamental distinction between varieties and species.

The general fertility of varieties is not enough to overthrow the view which I have taken with respect to the very general, but not invariable, sterility of first crosses and of hybrids, namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especially in the reproductive systems of the forms which are crossed.