Breeding barriers between Gossypium spp. and species of the Malvaceae family

Cotton breeders have long recognised the importance of alien germplasm from the Malvaceae family, especially the cultivated species, as sources of genes for cotton improvement. An understanding of the biological nature of the incompatibility systems that prevent hybridisation and/or seed development is necessary for the successful hybridisation and introgression between cotton and cultivated Malvaceae species (Hibiscus cannabinus and Abelmoschus esculentus). The objectives of the present study were to determine the reasons for reproductive isolation between Malvaceae species. The current study utilised two alien Malvaceae species and established that pollen–pistil incompatibilities are the primary reasons that hybrids with Gossypium hirsutum are not obtained. The alien pollen tubes showed major inhibition of growth in cotton pistils and seldom grew beyond the stigma. Only pollen tubes of A. esculentus grew into the ovary of cotton.

Genetics of Adaptive Radiation in Hawaiian and Cook Islands Species of Tetramolopium (Asteraceae). II. Genetic Linkage Map and Its Implications for Interspecific Breeding Barriers

In a study of the genetic mechanisms associated with adaptive radiation in Hawaiian Tetramolopium, a genetic linkage map was constructed in an interspecific cross. A total of 125 RFLP and RAPD markers were mapped into 117 different loci on nine linkage groups for a map length of 665.7 cM. Segregation distortion occurred in 49% of the mapped probes, located primarily in four linkage groups. High percentages of one parental species genotype (Tetramolopium rockii) were recovered in three of these blocks and the second parental species (T. humile) in the remaining block. The high degree of distorted segregation suggests the buildup of internal crossing barriers, even though island plant species are typically characterized as highly cross compatible with few to no internal crossing barriers. This work and a review of previous crossing studies in island plants show that internal (postmating) crossing barriers do exist. The weak crossing barriers have likely been overlooked because the main focus has been on diversification and speciation through adaptation to extremely diverse environments.