Introgressive Hybridization (1949)
Edgar Anderson

Chapter 1

Introgression in Iris: A Typical Example

Before we can discuss introgressive hybridization intelligently we must know what it is like. This first chapter attempts to define the phenomenon and then to give a description of one particular example. Detailed analysis of hybridization under natural conditions have shown that one of its commonest results is repeated backcrossing of the hybrids to one or both parents. With each successive backcross the partially hybrid nature of these mongrels becomes less apparent; the end result is an increased variability in the participating species. The possible importance of this gradual infiltration of the germplasm of one species into that of another was suggested by Ostenfeld in 1927. The process was specifically discussed in 1938 (Anderson and Hubricht) and named "introgressive hybridization." Its consequences were described as the "introgression" of one species into another, this terminology being deliberately chosen because it simplified the discussion of particular cases and avoided needless repetition. Introgression has since then been investigated in various genera of the higher plants, and its importance among the vertebrates has been demonstrated, at least for fishes and for Amphibia. Heiser has reviewed the literature on introgression critically (1949) and discussed its probable evolutionary and taxonomic significance.

Plate 1. Below: Flowers and enlarged sepals of Iris fulva (left) and Iris hexagona var. giganti-caerulea (right) to the same scales. Above: Map of area where these two species were hybridizing. H-1 and H-2 are the hybrid colonies diagrammed in Figs. 22 and 21, respectively.

For the purposes of this monograph one of the best examples of introgression is provided by two conspicuous irises of the Mississippi Delta. The scientific data concerning it are widely scattered in genetical, ecological, taxonomic, and horticultural literature, but when they are all assembled they agree, even to details. There can be little doubt that the interpretation presented below is as valid an explanation as one may ordinarily hope to find for complex natural phenomena. It has been studied taxonomically by Foster (1937), cytologically by Randolph (1934), genetically by Riley (1938, 1939a, 1939b), and ecologically by Viosca (1935). The evidence from Reed's experimental genetical analysis (1931) of a closely related cross has been confirmed by numerous horticulturists who have repeated the hybridization of the species from the Delta for garden purposes. Anderson has investigated the problem in both the field and the breeding plot. Riley, Foster, Viosca, and Anderson are in virtual agreement concerning the following account, though they have worked at different institutions and employed differing techniques.

The two species concerned, Iris fulva and Iris hexagona var. giganti-caerulea, [Author's note: Since these names are cumbersome and no generally accepted common names are available, they will be shortened to "Fulva" and "HGC" in the following discussion.] are strikingly different. In appraising the results of any hybridization, the problem is usually simplified if there are such conspicuous, manifold, clear-cut differences between the hybridizing entities as those which distinguish Fulva from HGC. The outstanding differences between these two species are presented in tabular form in Table 1, and a few details are illustrated in Plate 1. For those who have never seen these two irises, it is difficult to overemphasize how strikingly they differ. Though they cross easily and the hybrids have a considerable measure of fertility, they do not seem to be closely related. HGC is certainly more closely allied to Iris hexagona of the eastern seaboard and to Iris brevicaulis of the northern Mississippi Valley than to Fulva, from which it differs conspicuously in color, color pattern, size, habit, and ecological preferences. Fulva has smallish flowers of the color of old red brick; those of HGC are large with a brilliant pattern of dark blue, light blue, and white, set off by a signal patch of bright yellow. Its relatively few flowers are held crisply erect, whereas those of Fulva droop as if half wilted, one above the other, from successive internodes. Examination of the flowers reveals that Fulva has a red pigment over a yellow ground color; HGC, a blue pigment on a white ground.

Table I

Plant
Number
Tube
Color
Color of 
Sepal Blade
Sepal
Length
(cm)
Petal
Shape
Stamens Style
Appendages
Crest Index
Value
Percentage
of Pollen
Fertility
HGC
1
2
3
4
5
6
7
8
9
10
g
g
g
g
g
g
g
g
g
g
Pale violet-blue
Violet-blue
Violet-blue
Blue-violet
Pale blue-violet
Pale blue-violet
Pale violet-blue
Dark violet
Blue-violet
Blue-violet
9
9
9
9
10
9
11
9
9
9
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
g
17
16
17
17
17
17
17
16
17
17
95
94
97
95
94
96
92
92
89
98
Fulva
301
302
303
304
305
306
307
308
309
310
g
g
g
g
g
g
g
g
g
g
Red
Red
Red
Red
Pale Red
Red
Pale red
Red
Red
Red
5
6
6
6
7
6
6
6
6
6
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
f
0
0
0
0
1
0
0
0
0
0
98
97
99
95
95
99
98
95
99
97
Hybrid Colony H-1
101
102
103
104
105
106
107
108
109
110
i
g
i
g
i
i
g
i
i
i
Dark red-violet
Pale violet-blue
Red
Pale blue-violet
Red-violet
Very dark violet
Pale violet-blue
Pale violet
Dark red-violet
Dark red-violet
7
10
6
10
7
8
10
9
8
8
f
g
f
g
i
i
g
g
i
g
i
g
f
g
g
i
g
g
i
g
i
g
f
g
i
g
g
i
g
g
g
g
i
g
f
g
g
g
i
i
8
17
3
17
8
12
17
14
10
12
76
94
72
95
52
96
94
85
66
70
Hybrid Colony H-2
214
215
216
217
218
219
220
221
222
223
g
g
g
g
g
g
i
g
g
i
Violet-blue
Pale violet-blue
Blue-violet
Violet-blue
Pale violet-blue
Violet-blue
Dark red-violet
Blue-violet
Pale violet-blue
Dark violet
9
10
10
10
9
9
7
9
10
8
g
g
g
g
g
g
i
i
g
g
g
g
g
g
g
g
g
g
g
i
g
g
g
g
g
g
g
g
g
f
g
g
g
g
g
g
i
g
g
i
17
17
17
17
17
17
10
16
17
9
92
96
93
98
98
95
80
96
94
62

When HGC and Fulva are hybridized, the most conspicuous results are due to the recombinations of these two ground colors (and their various intermediates) with the two sap colors (and their intermediates). Although such hybrids have never been subjected to detailed genetic analysis, the cross has been repeatedly made for garden purposes by various hybridizers. The Bulletin of the American Iris Society from 1930 to 1945 contains frequent references to these and similar hybrids, occasionally with full descriptions of some of the segregates. Reed, however, has given a fairly detailed report (1931) on experimental hybrids between Iris brevicaulis and Fulva. Since I. brevicaulis is closely related to HGC (differing from it mainly in its low zigzag stem), Reed's results can be applied directly to the analysis of natural hybridization between Fulva and HGC, the more readily since they agree with those obtained by practical breeders.

As Reed's experimental results indicate (see in particular his colored Plate 1), bizarre recombinations are formed in the second generation and in backcrosses when the pigment genes segregate more or less independently of the ground-color genes. The differences between red pigment vs. blue and white ground color vs. yellow each seem to be multifactorial, so that for the first we get a whole series from blue to purple to red, and for the second a similar transition from white to ivory to light yellow to bright yellow. In the second generation we may get a blue pigment more or less like that of HGC on top of a yellow ground color; the result will be a flower with soft tones of ashy gray. At the other extreme we may get the red of Fulva over the white ground color of HGC, resulting in a delicate rose pink. HGC, furthermore, varies from plant to plant in the strength of its blue pigment, some plants being practically albinos. If this extreme is carried over into a hybrid, the resulting flower may be largely yellow or ivory, depending on its underlying ground color. Along with these recombinations of the color genes go various degrees of intermediacy between the large flowers of HGC and the small ones of Fulva, between Fulva's floppy petals and the upright ones of HGC. Undoubtedly, there must as well be segregation for some of the basic physiological differences that limit Fulva prevailingly to one kind of a situation, HGC to another.

Fulva is a wide-ranging species growing in wet clay, soils from the Wabash and Ohio River valleys down to the lower delta of the Mississippi. Characteristically it is found in the flat valleys of these large rivers along the edges of the natural levees that they build for themselves. It seems to prefer semishade and very often grows along drainage ditches. HGC never gets far from the sea; it is a plant of the lower delta and is found in full sun in the mucky soil of tidal marshes, where the soil is never acid and may be quite alkaline.

The area where these two species come into contact is, therefore, the lower Mississippi Delta, mostly in the region between New Orleans and the sea. It is flat country where differences of a few inches in the height of the land have more effect on the vegetation than hundreds of feet might have in other parts of the world. (Viosca, 1935.) Here, for thousands of years, the river has been building its delta, splitting itself up into numerous weaving branches, which change their courses constantly and sometimes catastrophically. In those rare portions of this rich agricultural region in which man has not greatly altered the natural pattern of the vegetation, Fulva and HGC come into contact whenever a natural levee penetrates a marsh, as, for instance, when a shifting bayou cuts across the course of an abandoned deltaic stream. At such places a few hybrids are sometimes to be found where a natural levee runs into a wide tidal marsh. Hybridization between Fulva and HGC must have been going on occasionally for a very long time. The whole pattern of relationship between these two species, however, has been greatly changed by human occupation. The delta region was settled mainly by the French, and for more than a century little French farms have lined the rivers and bayous. Property lines run straight back at right angles to the rivers. Each family's holding is long and narrow, so that all through the countryside the houses are close together. There has been little large-scale fanning. The whole covering of natural vegetation has not been wiped clear as in much of the cotton belt. The average family has cleared some lands for fields, left others in pasture, and has kept a good deal of woodland from which small amounts of cordwood and timber are cut from nine to nine.

This outline of the two species and the environment in which they meet presents the two fundamentals of the Fulva-HGC interaction on the Mississippi Delta: (1) The two strikingly different but interfertile species, (2) largely kept apart by dissimilar natural environments, progressively altered in part by thousands of small farmers, no two of whom treated their small holdings in exactly the same fashion but few of whom obliterated entirely the natural vegetation. By the early 1900's observant local naturalists were beginning to comment on the results. From New Orleans southward, in many a small community there would be cow pasture brilliant with many-colored irises, white, yellow, wine-colored, red, and blue, many of them so attractive that they were moved into nearby gardens. Eventually, Dr. John K. Small of the New York Botanical Garden, called them to the attention of botanists and iris gardeners, illustrating them in full color and describing them as species new to science (1927; Small and Alexander, 1931). From the first, both among botanists and iris gardeners, there were those who suggested that the whole complex was of hybrid origin and eventually Viosca's careful ecological survey of the problem convinced all but a few. Foster came to the same conclusions independently on taxonomic and cytological evidence, and Riley's investigations confirmed and extended those of Viosca. Meanwhile, the horticultural world took a deep interest in the beautiful chance hybrids of these remote pastures. Hardier and more generally satisfactory hybrids eventually were bred artificially, but until these manmade hybrids reached the market in quantity there was a brisk local business in the brilliant mongrel iris populations of these little agricultural communities of the lower delta.

Riley's intensive studies (1938, 1939a, 1939b) of these hybrids were made at one of the localities where Viosca had discovered a particularly brilliant group. An old, abandoned deltaic stream had built up two levees, one of which served as a base for the public road. One of the bayous of the river had swung out, cutting across the ancient ridges and forming a wide marsh in which there were numerous plants of HGC. Fulva occurred sporadically along the edge of the abandoned stream for several miles along the road. At the very point where these two habitats met, there was a series of small, neighboring farms, their property lines stretching back at right angles to the road and the abandoned natural levee. Each family had managed its property a little differently, and the holdings were all so narrow that the whole community was almost like a Iaboratory experiment. At several places there were occasional iris plants that were typical of neither Fulva nor HGC and might possibly have been of partially hybrid origin. On one farm, however, there were great numbers of peculiar irises, most of them resembling the hybrids obtained by the iris breeders from controlled pollinations. They grew in two main-groups (H-1 and H-2 in Plate 1). The H-2 group was rather similar to HGC, and some of its members were within the variation range of that species. On the whole they looked like a population of HGC slightly more variable than usual, but if one tabulated the variation it was mostly in the direction of Fulva. That is to say, the flower colors tended a little more towards red on the average; there were more small flowers; there were more frequently several flowers on a stalk; and the petals were not all held so stiffly upright as on a typical HGC. The H-1 group was a brilliant mixture. It varied from plants looking more or less like HGC to others resembling the artificial F1 to a few others more like Fulva. The flowers were large on some plants, small on others. Petal and sepal shape differed from plant to plant. The colors ranged from deep blue to red, with many variations in the size, shape, color, and pubescence of the signal patch. The spot at which this hybrid swarm was growing was the abandoned bed of the old deltaic stream. On this particular farm the land had mostly been cleared, and then a second-growth woodland had been allowed to come up in the depression. This had again been cut over heavily, and the whole area had been overpastured. So many cattle had been kept on the area that the shrubs in the swamp had been browsed. There was much bare soil and relatively little grass, and in the softer ground of the swamp the cattle had created "buffalo wallows" by their attempts to get through in wet weather. On the adjoining farms the overpasturing was not so evident. The woods on one had been almost entirely cleared from the depressions and replaced by a healthy stand of grass. On the other, the second-growth woodlot had been preserved with little cutting over and very little pasturing.

These facts are described in such detail because this particular case is a really critical experiment for understanding the dynamics of hybridization. The bizarre hybrid swarm was entirely limited to this greatly disturbed area. On one side the hybrid plants went up to the very fence line of the adjoining property but no farther. On the other side they did not quite extend to the fence line. In this little bit of repeatedly cut-over and heavily pastured woodland, adjacent to the spot at which the two species were in contact, there were many more hybrids than in all the rest of the vicinity put together. The reasons for this connection between the disturbance of the habitat and the results of hybridization will be discussed in the next chapter; for the present it needs to be pointed out merely that such a connection is typical of many of the instances of hybridization that have been carefully studied in the field.

Riley made population samples of Fulva, HGC, and various hybrid colonies. Table I shows the kind of basic data that he obtained from a colony of HGC, a colony of Fulva, and the two hybrid colonies H-1 and H-2. For each plant be recorded whether it was essentially like HGC, like Fulva, or intermediate in its tube color, petal shape, stamen exsertion, style appendages, and shape of crest. He also measured the sepal lengths, recorded the ground color of the sepal with the aid of a standard color chart, and determined the percentage of fertile pollen in each plant. Table 1 shows the kind of results he obtained for ten plants from each of the four colonies. HGC is essentially uniform in all these characters. Fulva was similarly uniform, varying only in whether the plants were red or pale red. Scored by the same method, the two hybrid colonies presented a very different picture and (a most important point) they showed significant differences between themselves. Both of them varied from plant to plant, but the variation in Colony H-1 was many times as striking. It varied in its extremes for each character and in its combinations of characters. It will be noted that there are no two plants with exactly the same combination of characters.

Colony H-2 was much more uniform. Some of its plants were indistinguishable from HGC; others showed a few slight differences on close scrutiny; a few were clearly intermediate; and, in such measurable characters as sepal length, the population as a whole is slightly more like Fulva than HGC normally is.

Table 1 shows that variation in fertility parallels the morphological variation. Fulva and HGC have pollen of high fertility; there is more sterility in the hybrid colonies, and much more in H-1 than in H-2.

To summarize all these facts in a rough kind of way, Riley used a method originated by Anderson (1936d) which is described and discussed in Chapter 6. He arbitrarily assigned values to the seven morphological characters recorded in Table 1 and set the scores in such a way that resemblance to HGC was always high in value and resemblance to Fulva low in value. This procedure produced an index running from 0 to 17. The calculated index values for the ten representative plants are shown in Table 1. In his Fig. 3 the combined scores for all the plants of each colony were shown graphically. The plants of Fulva have uniformly low values; those of HGC are uniformly high. Colony H-2 is much like HGC but has a slight trend in the direction of Fulva. Colony H-1, on the other hand, is in general a mixture of everything from intermediates to plants closely resembling HGC.

The presentation of Table I and Plate 1 completes the description of hybridization between Fulva and HGC. In succeeding chapters we shall discuss the ways in which the results of interspecific hybridization are controlled by the dynamics of the environment, by the dynamics of the germplasm, and by the interactions of these forces in actual populations. We shall continue to refer to this example. It has been well documented by Viosca and by Riley (in addition to the papers cited above, there are others on pollen fertility and on developmental rates). It serves the better as illustrative material because it demonstrates features that we shall notice again and again when other examples of hybridization are described in detail: (1) the relation between the effects of hybridization and man's disturbance of the habitat, (2) the differences between various hybrid populations made between the same species and in the same region, (3) the predominance of mongrels of partially hybrid ancestry which closely resemble one of the participating species.

Bibliography