EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



711 



have long been favorite subjects for investi- 

 gation and comment. In the following dis- 

 cussion it will be seen that varying degrees 

 of mutualism exist, from a slight benefit to 

 a remarkable interdependence of both spe- 

 cies in the partnership. The illustrations 

 are arranged in a series from plant-plant 

 relations through plant-animal to animal- 

 animal mutuahsm. 



Nitrogen-fixing bacteria {Rhizobiitm ra- 

 dicicola and R. leguminosarum) are asso- 

 ciated with the legumes in which they 

 produce gall nodules on the roots, and the 

 extensive experience of the agriculturists 

 gives ample proof that this relationship is 

 of great value to the growth of both the 

 legumes and the plants that may later be 

 grown in the same field. As a result, a 

 leguminous species is usually included as 

 one of the plants in the practice of crop 

 rotation. 



A mutual interdependence of bacteria 

 and legumes is suggested (Wilson, 1940, 

 p. 86). In spite of this close association, 

 and in spite of the benefit to both the leg- 

 umes and the bacteria, it is not yet certain 

 that both the host species and the species 

 of bacteria have a modified germinal sys- 

 tem produced by natural selection as a re- 

 sult of the benefits of their mutualistic re- 

 lationship. That there has been an evolu- 

 tionary adaptation on the part of the bac- 

 teria is suggested by the fact that they 

 seem to be usually associated with leg- 

 umes. The bacteria are found free in the 

 soil, but the fixation of nitrogen occurs only 

 when they are associated with the host 

 plant. 



Nitrogenous compounds are excreted by 

 the root nodules during early stages of their 

 development as aspartic acid and 

 alanine, together with small amounts of the 

 oxime of oxalacetic acid. Nitrogen is prob- 

 ably fixed as hydroxylamine, which con- 

 denses with the oxalacetic acid produced 

 by the host plant. The oxime is reduced to 

 aspartic acid, wliich then serves as a start- 

 ing material for the synthesis of other 

 amino acids. It is presumed that the /3- 

 alanine arises by decarboxylation of the 



It may be assumed that the legumes 

 have probably exerted selection pressure on 

 the bacteria. Strains of bacteria are known 

 that differ from one another in their host 

 relations and in their immunological char- 

 acteristics. Some nodule-producing strains 

 of Rhizobium are ineffective in benefiting 

 their legume host plant ( Nichol and Thorn- 

 ton, 1941). The abiUty to compete effec- 

 tively in the soil with others is independ- 

 ent of the degree of effectiveness of nitro 

 gen fixation. The strain with the higher 

 initial growth rate may completely check 

 reproduction of others outside the plant, 

 and in some plants with rapid root growth 

 (peas and soybeans), the dominant strain 

 in the soil may rapidly form all the nodules 

 up to the nodule-producing limit of the 

 plant. In plants with slow root growth 

 (clover), the first nodules do not inhibit 

 the development of later nodules formed by 

 the same or a different strain of bacteria. 

 Infectivity differs in different strains. The 

 roots secrete a substance that serves as a 

 source of energy, or an accessory growth 

 substance that stimulates the growth of at 

 least some strains of bacteria. 



So far as can be seen, the benefit to the 

 legumes is hardly much more than the 

 benefit to other plants partaking of the soil 

 nutriment, and it has not been demon- 

 strated that such an advantage might not 

 accrue from a chance effect. The growth of 

 the gall may be caused by agents in the 

 bacteria, and not be an adaptive response 

 on the part of the plant (see pp. 496-498). 



Lichens have been used as a classical 

 example of mutualism. Various species of 

 fungi and algae five together in such a 

 manner that lichen taionomists treat the 

 system as if it were a single species. The 

 fungus partner is usually a species found 

 only in a lichen association and is practi- 

 cally a parasite on the alga (Tschermak, 

 1941). All the species of algae, however, 

 can live independently of the fungus asso- 

 ciation, so that there is no question that the 

 fungus is by far the more dependent part- 

 ner. Through symbiosis with the fungus, 

 the alga is enabled to exist in places that 

 would otherwise be uninhabitable, such as 



aspartic acid. 



•Fixation of the nitrogen (Quastel, 1947) may thus be expressed: 



Nj + methaemoglobin (Fe'") ;;=i NH2OH -f haemoglobin (Fe") 

 NH.OH -t- COOH.CO.CH..COOH ^COOH.C(NOH).CH2.COOH -f- H,0 

 (Hydroxylamine) (Oxalacetic acid) (Oxalacetic oxime) 



COOH.C(NOH).CH..COOH > COOH.CH(NH0.CH2.COOH 



(Oxalacetic oxime) Reduction (Aspartic acid) 



(1) 

 (2) 



^3) 



