Cross compatibility in intraspecific and interspecific hybridization in yam (Dioscorea spp.)

Abstract

Yam (Dioscorea spp.) is a staple crop for millions of people in the tropics and subtropics. Its genetic improvement through breeding is being challenged by pre-zygotic and post-zygotic cross-compatibility barriers within and among species. Studies dissecting hybridization barriers on yam for improving the crossability rates are limited. This study aimed to assess the cross-compatibility, which yielded fruit set, viable seeds and progeny plants in an extensive intraspecific and interspecific crossing combinations in a yam genetic improvement effort to understand the internal and exogenous factors influencing pollination success. Cross-compatability was analyzed at the individual genotype or family level using historical data from crossing blocks and seedling nurseries from 2010 to 2020 at the International Institute of Tropical Agriculture (IITA). The average crossability rate (ACR) was lower in interspecific crossing combinations (6.1%) than intraspecific ones (27.6%). The seed production efficiency (SPE) values were 1.1 and 9.3% for interspecific and intraspecific crosses, respectively. Weather conditions and pollinator's skills are the main contributors to the low success rate in the intraspecific cross combinations in yam breeding. At the same time, genetic distance and heterozygosity played little role. Interspecific cross barriers were both pre-zygotic and post-zygotic, resulting from the evolutionary divergence among the yam species. Dioscorea rotundata had higher interspecific cross-compatibility indices than D. alata. Distant parents produced intraspecific crossbred seeds with higher germination rates compared to closest parents (r = 0.21, p = 0.033). This work provided important insights into interspecific and intraspecific cross-compatibility in yam and suggested actions for improving hybridization practices in yam breeding programs.

Optimum time for hand pollination in yam (Dioscorea spp.)

“…All these materials were breeding lines maintained by the IITA Yam Breeding Unit (S1 Table ). The cross-compatibility among selected genotypes and their ploidy statuses were based on historical data information (Mondo et al, 2022).…”

“…However, the optimum time for yam pollination is location-specific and depends on local environmental conditions (Mondo et al, 2020). High relative humidity, well-distributed rainfall, sunshine, and moderate atmospheric temperatures are the leading climatic factors for successful pollination in D. alata and D. rotundata yams (Abraham and Nair, 1990; Mondo et al, 2022). Recommended time for hand pollination in Nigeria (12 noon-3 p.m.) was set ~40 years ago (Akoroda, 1983), thus, there is a chance that trends recorded four decades ago may have changed.…”

“…However, improved cultivar development through breeding in yam is challenged by sexual reproduction abnormalities resulting from sparse, irregular, and asynchronous flowering, cross compatibility barriers as the vegetative propagation is favored at the expense of botanical seeds during the domestication and subsequent cultivation process (Mondo et al, 2020). Yam plant exhibits extremely low levels of fruit-to-flower and seed-to-ovule ratios, partly because of the sensitivity of its reproductive phases to suboptimal weather conditions (Mondo et al, 2022). Important climatic factors such as temperature, rainfall, relative humidity, and light intensity fluctuate from one location to the other and even from time to time in the same location.…”

“…Important climatic factors such as temperature, rainfall, relative humidity, and light intensity fluctuate from one location to the other and even from time to time in the same location. These fluctuations dictate the limit within which controlled pollination can be successfully conducted in any given location for a given species (Salami, 2016; Maity et al, 2019; Raina and Kaul, 2019; Katano et al, 2020; Mondo et al, 2022).…”

“…Most yam species, including D. alata and D. rotundata , are dioecious with male and female flowers on separate individuals (Agre et al, 2020; Mondo et al, 2020; 2021a; 2022). The gene flow between and among these species to meet breeding objectives depends, therefore, on crosspollination success.…”