Inbreeding, heterosis, fertility, plasmon differentiation and Phytophthora resistance in Solanum verrucosum Schlechtd., and some interspecific crosses in Solanum

Solanum verrucosum (2n = 24) is a self-compatible, highly fertile species. This conclusion is drawn from the results of studies on 21 various introductions of this species. In spite of self-compatibility and good male and female fertility, the species generally needs a pollinating agent to ensure seed set owing to the supra-staminal pistil characteristic of its flowers.S. verrucosum is heterozygous for genes controlling qualitative as well as quantitative characters. Differences between and variation within introductions of S. verrucosum occur. The species is not resistant to inbreeding. Pre-breeding and selection within the species is demonstrated to be effective and therefore is advisable for its efficient utilization in potato breeding.Within S. verrucosum , F 1 hybrids show a better performance than inbreds. The segregating F 2 show inbreeding depression. Heterosis is more pronounced in early growth stages. In different characters, magnitude of heterosis in F 1 and degree of inbreeding depression in F 2 are associated.In S. verrucosum , 'tall stem' is controlled by two complementary dominant genes, as is obvious in hybrids between the short-stemmed introduction CPC 1339 and the tall introductions CPC 2247 and PI 195172-237-9-13. A similar genetic basis determines early v. late flowering in CPC 1339 x PI 195172-237-9-13, late flowering of CPC 1339 being recessive. A significantly negative correlation has been found between flowering time and stem height.The 'bubble sterility' (pollen showing breakdown of the cytoplasm, staining faintly and looking bubbled) occurring in S. verrucosum Haw 2246 is controlled by polygenes. The 'blunt spine walls' of pollen grains (pollen with many coarse blunts on their walls), discovered in the same introduction, behaves as a monogenic recessive character. Both female fertility (number of seeds per berry) and male fertility (percentage of stainable pollen) are found from the study of different populations to be under polygenic control.Colchicine induced S. verrucosum plants with 48 chromosomes show a decrease in male and female fertility but an increase in vigour.In S. verrucosum , grafting onto tomato improves seed set and results in richer flowering and in a more vigorous growth, also in some interspecific hybrids. Grafting has no significant effect on male fertility, either in S. verrucosum , or in the malesterile interspecific hybrids.Upon inoculation with Phytophthora infestans Races 4, 1.4, 1.2.3.4, 1.3.4.7.8 and Isolates B 19 and 331, S. verrucosum reacts in different ways. Several introductions have segregated for resistance, others have appeared to consist of either susceptible or almost resistant individuals only. Both major genes and polygenes are assumed to condition resistance. Selection appears effective.Whether by direct approaches (the use of self-set seed) or indirect (the introduction of self-compatibility), haploids (parthenotes with 2n = 24 from S. tuberosum ) and self- incompatible species, especially S. phureja , proved to be highly heterozygous and suffer greatly from inbreeding. Worthless plants, and segregation for several vegetative and generative characters occur in inbred populations from self-incompatible species. In some cases, the degree of inbreeding is associated with the recessivity of specific genes.The self-compatible species S. polyadenium and S. etuberosum are neither crossable mutually, nor with any of the other species studied.Though barriers reducing berry and seed set and hybrid seed germination are present, S. verrucosum is easily crossed with several other species and haploids, but usually only as a female partner: as a rule pollen from self-compatible species is inhibited in styles of self-incompatible ones (unilateral incompatibility).Hybrids between female S. verrucosum and haploids and diploid species generally grow well. In two cases androgenetic plants (2n = 24) have been found among hybrid populations, and in one case a parthenogenetic seed (2n = 24) of S. verrucosum has occurred, giving rise to homozygous and homogeneous population. The parthenogenetic population shows reduced vigour but improved seed set and tuber yield and all its plants are resistant to Phytophthora Race 1.2.3.4.All interspecific hybrids with different female S. verrucosum introductions are highly male sterile due to plasmon-genic interaction. The dominant plasmon-sensitive genes are wide spread in haploids and diploid species. In the 21 introductions of S. verrucosum , three plasmons, [Tr s], [Ps s] and [SY s], occur, each leading to diagnostic appearance of male sterility in the presence of the corresponding Tr , Ps and Sv dominant plasmon- sensitive genes. The male sterilities are 'tetrad sterility' (generally poor shedding and clumping of the faintly or non-stained sterile pollen grains in tetrads) in [Tr s] plasmon, 'partly stained pollen sterility' (pollen production normal, but the sterile pollen does not show complete staining) in [Ps s] plasmon, and 'striped vacuolar sterility' (very poor shedding of faintly stained sterile pollen, characterized by the presence of vacuoles and stripes) in [Sv s] plasmon. All S. verrucosum introductions possess sterilizing (sensitive) plasmons, contrary to a report in literature.In the backcrosses and segregating populations of the interspecific hybrids with S. verrucosum as a female, two additional types of male sterility have occurred: 'undivided microsporocytes' (failure of pollen mother cells to separate into 4 cells, irrespective of nucleus division; consequently microsporocytes are shed, each possessing 4 nuclei) and 'non-shedding' (pollen sacs hardly shedding any pollen). The former type (also discovered in pure S. verrucosum ) is controlled by two complementary recessive genes, the latter by the presence of three complementary recessive ns genes in the [ns s] plasmon of S. verrucosum .The 'lobed appearance' of pollen grains (each grain having three lobes) is conditioned by dominant Ld gene(s) in [Ld s] plasmon of S. verrucosum. Generally plants having [Tr s] plasmon also possess [Ld s] plasmon. The Ld genes are wide spread, like the dominant Tr plasmon-sensitive genes which lead to male sterility. That is why tetrad sterility and lobed pollen are mostly associated in F 1 hybrids. Perhaps this is the reason why the lobed appearance of pollen has been reported by some authors as a type of male sterility. In our studies lobed pollen and male sterility have proved to be different characters. In one cross, duplicate linkage groups between Tr and Ld genes have been discovered with a crossing over value of 15 %.In all the above cases of plasmon-genic interaction, S.verrucosum possesses 'sensitive' plasmons, whereas other species and haploids are assumed to have 'resistant' plasmons.Self- and cross-incompatibility behaviour in S.phureja and S. stenotomum x S. phureja are controlled by two loci, S and R . The incompatibility reaction is gametophytically controlled. The S locus is epistatic to R ; the latter has an allele ( R fi ) which, when present in homozygous condition, leads to female incompatibility. Irregularities observed in the studies of incompatibility may be due to recombining modifier complexes.Some evidence is presented in the discussion suggesting that the basic chromosome number in Solanum may be less than 12.The evolutionary aspects of self-compatibility and unilateral incompatibility is discussed. A two power competition hypothesis is presented to account for the relation, crossability and the expected behaviour of hybrids between self-compatible and selfincompatible species. Specific genes are assumed to be present in self-incompatible species, preventing fertilization by pollen carrying the self-compatibility alleles ( S c ). The hypothesis has shown why such genes must have developed in the course of evolution, what the result of their functioning has been and how self-compatible species react toward their development.Data from literature on the crosses between self-compatible and self-incompatible species (with the gametophytic system of incompatibility) from several genera of different families have been critically considered and are easily explained on the basis of this new hypothesis.Breeding for resistance to Phytophthora has been discussed. A new hypothesis is presented to combine 'uniform' and 'differential' resistance. Both types of resistance are assumed to be controlled by specialized interacting genes for resistance. The polygenes are 'older' and control the 'primary general' resistance, the major genes govern the direct (switching) work towards specific races.Breeding at the diploid level and the use of S. verrucosum and overcoming the barriers to its use in breeding are debated.