It is 100 years since the great Walter T. Swingle (Cooper et al., 1962; Tasker, 2010) first attempted to hybridize Citrus and Citropsis (Swingle, 1913). He had established the Citropsis genus (Swingle, 1914) by removing African species from Limonia L. recognizing them as “closely and clearly related to Citrus” and by this time had already envisioned their potential for citriculture and attempted crosses onto Citrus aurantiifolia (Christm.) Swing. However, after a breeding career of more than 50 years, and success in hybridizing a great many other citrus relatives, he concluded that Citrus and Citropsis were sexually incompatible (Swingle, 1945). Many other breeders (e.g., Barrett, 1977; Ford and Peder, 1969; Hutchison, 1976; Iwamasa et al., 1985, 1988) have subsequently attempted this cross, motivated by the useful traits that Citropsis could bring to commercial citriculture, but no hybrid seedlings were obtained. Somatic hybrids between Citrus and Citropsis were first reported by Grosser et al. (1990) and the growth problems exhibited by these initial two hybrids were subsequently overcome by expanding the program to include a greater range of parents (Grosser et al., 1996). Indeed, a ‘Nova’ mandarin (Citrus reticulata Blanco + Citropsis gilletiana Swing. & M.Kell.) somatic hybrid has grown with good vigor and shows some promise as a rootstock (Grosser and Chandler, 2003). However, none of these hybrids, or those from other Citrus + Citropsis somatic hybridization programs, have ever flowered despite their considerable age (Grosser and Chandler, 2003; Guo and Deng, 2001; J.W. Grosser, personal communication). The breakthrough came in 2006 when Japanese breeders reported the first intergeneric sexual hybrids between Citrus and Citropsis (Yahata et al., 2006) produced by combining conventional hybridization with embryo rescue and seedling grafting. Although their hybrids have been slow-growing and are yet to flower (H. Kunitake, personal communication), they demonstrate a useable level of sexual compatibility between Citrus and Citropsis and were the impetus for renewing our attempts at this cross.
Intergeneric hybridization represents an opportunity to combine genomes from distinctly different plants and to introgress traits not found in the main genus of interest. However, plant groups differ markedly in their crossability with prezygotic and postzygotic barriers to hybridization becoming more severe as the distance between taxa increases (Ladizinsky, 1992). Conversely, success in intergeneric hybridization can sometimes be attributed to taxonomic anomalies rather than the sexual union of very distant parents. Such is the case with citrus relatives where reported “intergeneric” hybrids involving Poncirus Raf., Microcitrus Swing., Eremocitrus Swing., Fortunella Swing., and Citrus (e.g., Barrett, 1977; Hutchison, 1976; Iwamasa et al., 1985, 1988) would constitute only “interspecific” hybrids under a more conservative taxonomic treatment supported by many recent molecular studies such as Bayer et al. (2009). This molecular evidence reaffirms Citrus [broad circumscription combining all six genera of “true citrus fruit trees” (Swingle and Reece, 1967)] and Citropsis as distinct genera. Considered within the context of this broad circumscription of Citrus, there have been only two successful attempts at intergeneric sexual hybridization within Aurantioideae: the work of Hisato Kunitake and colleagues with Citrus and Citropsis, described previously, and the often overlooked success of Medina et al. (1998) in hybridizing Citrus and Severinia Ten. ex Endl. (syn. Atalantia Corrêa). Both programs have resulted in only small numbers of hybrids with no reported flowering.
Although Citropsis has been little studied, certain species are known to posses traits of potential value to modern citriculture (Krueger and Navarro, 2007) including resistance to burrowing nematode [Radopholus citrophilus Huettel, Dickson & Kaplan (Ford and Feder, 1960)] and foot rot [Phytophthora nicotianae Breda de Haan (Swingle and Reece, 1967)]. It was one of the better performing citrus relatives in the extensive rootstock work of Bitters et al. (1977) in California but fell short of expectations in Florida because of too many unfavorable attributes (Barrett, 1977) including cold tenderness and bud union incompatibility (Ford and Peder, 1969). In our own rootstock experiments in subtropical Australia, 10-year-old ‘Eureka’ lemon [Citrus limon (L.) Burm. f.] on Citropsis schweinfurthii (Engl.) Swing. & M.Kell. and Citropsis gilletiana are healthy compact trees surviving under high Phytophthora de Bary pressure. Recognizing that Citropsis species fall short of commercial acceptability in their own right, some breeders have used protoplast fusion in an attempt to capture their useful traits by combining them with complementary Citrus species for use as rootstocks (Grosser and Chandler, 2003; Grosser et al., 1990).
Despite these past efforts, a breeding methodology for the efficient introgression of useful traits from Citropsis without the transfer of economically undesirable characteristics has yet to be demonstrated. To address this problem, we used diverse germplasm in an attempt to efficiently generate fruitful intergeneric hybrids. Siblings of species within the Citrus and Citropsis genera were control-crossed and we report the outcomes here.
Barrett, H.C. 1977 Intergeneric hybridization of Citrus and other genera in citrus cultivar improvement. Proc. 3rd Intl. Citrus Congr. p. 586–589
Bayer, R.J., Mabberley, D.J., Morton, C., Miller, C.H., Sharma, I.K., Pfeil, B.E., Rich, S., Hitchcock, R. & Sykes, S. 2009 A molecular phylogeny of the orange subfamily (Rutaceae: Aurantioideae) using nine cpDNA sequences Amer. J. Bot. 96 668 685
Belea, A. 1992 Interspecific and intergeneric crosses in cultivated plants. Akadémiai Kiadó, Budapest, Hungary
Bitters, W.P., Cole, D.A. & McCarthy, C.D. 1977 Citrus relatives are not irrelevant as dwarfing stocks or interstocks for citrus. Proc. 3rd Intl. Citrus Congr. p. 561–567
Cheng, F., Haihua, D. & Xiwen, C. 2003 Research and utilization of Erianthus arundinaceus at Hainan Sugarcane Breeding Station Sugarcane Canesugar 6 1 14
Couturon, E., Lashermes, P. & Charrier, A. 1998 First intergeneric hybrids (Psilanthus ebracteolatus Hiern × Coffea arabica L.) in coffee trees Can. J. Bot. 76 542 546
Falk, D.E. & Kasha, K.J. 1981 Comparison of the crossability of rye (Secale cereale) and Hordeum bulbosum onto wheat (Triticum aestivum) Can. J. Genet. Cytol. 23 81 88
Ford, H.W. & Feder, W.A. 1960 Citropsis gilletiana, a citrus relative resistant to the burrowing nematode in laboratory tests Proc. Florida State Hort. Soc. 73 60 64
Ford, H.W. & Peder, W.A. 1969 Development and use of citrus rootstocks resistant to the burrowing nematode, Radopholus similis. Proc. 1st Intl. Citrus Congr. p. 941–948
Forster, P.I. & Smith, M.W. 2010 Citrus wakonai P.I.Forst.&M.W.Sm. (Rutaceae), a new species from Goodenough Island, Papua New Guinea Austrobaileya 8 133 138
Grosser, J.W. & Gmitter, F.G. Jr 1990b Somatic hybridization of Citrus with wild relatives for germplasm enhancement and cultivar development HortScience 25 147 151
Grosser, J.W. & Gmitter, F.G. 2011 Protoplast fusion for production of tetraploids and triploids: Applications for scion and rootstock breeding in citrus Plant Cell Tissue Organ Cult. 104 343 357
Grosser, J.W., Gmitter, F.G. Jr, Tusa, N. & Chandler, J.L. 1990 Somatic hybrid plants from sexually incompatible woody species: Citrus reticulata and Citropsis gilletiana Plant Cell Rep. 8 656 659
Grosser, J.W., Mourao-Fo, F.A.A., Gmitter, F.G. Jr, Louzada, E.S., Jiang, J., Baergen, K., Quiros, A., Cabasson, C., Schell, J.L. & Chandler, J.L. 1996 Allotetraploid hybrids between Citrus and seven related genera produced by somatic hybridization Theor. Appl. Genet. 92 577 582
Guo, W.W. & Deng, X.X. 2001 Wide somatic hybrids of Citrus with its related genera and their potential in genetic improvement Euphytica 118 175 183
Hermsen, J.G.T. 1992 Introductory considerations on distant hybridization, p. 1–14. In: Kalloo, G. and J.B. Chowdhury (eds.). Distant hybridization of crop plants. Springer-Verlag, Berlin, Germany
Iwamasa, M., Nito, N., Katayama, Y., Yamaguchi, T. & Matsunaga, S. 1985 Cross compatibility between plants of the Aurantioideae Bul. Faculty Agr. Saga Univ. 59 57 69
Iwamasa, M., Nito, N. & Ling, J.T. 1988 Intra- and intergeneric hybridization in the orange subfamily, Aurantioideae. Proc. 6th Intl. Citrus Congr. p. 123–130
Khush, G.S. & Brar, D.S. 1992 Overcoming the barriers in hybridization, p. 47–61. In: Kalloo, G. and J.B. Chowdhury (eds.). Distant hybridization in crop plants. Springer-Verlag, Berlin, Germany
Krueger, R.R. & Navarro, L. 2007 Citrus germplasm resources, p. 45–140. In: Khan, I.A. (ed.). Citrus genetics, breeding and biotechnology. CABI, Wallingford, UK
Ladizinsky, G. 1992 Crossability relations, p. 15–31. In: Kalloo, G. and J.B. Chowdhury (eds.). Distant hybridization in crop plants. Springer-Verlag, Berlin, Germany
Medina, F.-H.P., Bordignon, R. & Ballve, R.M.L. 1998 Sunkifolias and Buxisunkis: Sexually obtained reciprocal hybrids of Citrus sunki × Severinia buxifolia Genet. Mol. Biol. 21 129 133
Pan, A.D. 2010 Rutaceae leaf fossils from the Late Oligocene (27.23 Ma) Guang River flora of northwestern Ethiopia Rev. Palaeobot. Palynol. 159 188 194
Reed, S.M., Jones, K.D. & Rinehart, T.A. 2008 Production and characterization of intergeneric hybrids between Dichroa febrifuga and Hydrangea macrophylla J. Amer. Soc. Hort. Sci. 133 84 91
Scora, R.W. 1988 Biochemistry, taxonomy and evolution of modern cultivated citrus. Proc. 6th Intl. Citrus Congr. p. 277–289
Swingle, W.T. & Reece, P.C. 1967 The botany of citrus and its wild relatives, p. 190–430. In: Reuther, W., H.J. Webber, and L.D. Batchelor (eds.). The citrus industry: Vol. 1. History, world distribution, botany and varieties. Univ. of California, Berkeley, CA
Wang, L., Cai, Q., Fan, Y., Lu, X., Aitken, K., Ma, L., Liu, X. & Xia, H. 2007 Study on the distant hybrid utilization between Saccharum and Erianthus arundinanceus Southwest China J. Agr. Sci. 20 721 726
Yahata, M., Kunitake, H., Yasuda, K., Yamashita, K., Komatsu, H. & Matsumoto, R. 2006 Production of sexual hybrid progenies for clarifying the phylogenic relationship between Citrus and Citropsis species J. Amer. Soc. Hort. Sci. 131 764 769
Yasuda, K., Kunitake, H., Yahata, M. & Matsumoto, R. 2010 Investigation of sexual intergeneric hybrid progenies between citrus cultivars and Citropsis schweinfurthii. Proc. 11th Intl. Citrus Congr. p. 121–124