Reproductive Isolation Among Three Nocturnal Moth-Pollinated Sympatric Habenaria Species (Orchidaceae)

Abstract

Comparison and quantification of multiple pre- and post-pollination barriers to interspecific hybridization are important to understand the factors promoting reproductive isolation. Such isolating factors have been studied recently in many flowering plant species which seek after the general roles and relative strengths of different pre- and post-pollination barriers. In this study, we quantified six isolating factors (ecogeographic isolation, phenological isolation, pollinator isolation, pollinia-pistil interactions, fruit production, and seed development) that could possibly be acting as reproductive barriers at different stages among three sympatric Habenaria species (H. limprichtii, H. davidii, and H. delavayi). These three species overlap geographically but occupy different microhabitats varying in soil water content. They were isolated through pollinator interactions both ethologically (pollinator preference) and mechanically (pollinia attachment site), but to a variable degree for different species pairs. Interspecific crosses between H. limprichtii and H. davidii result in high fruit set, and embryo development suggested weak post-pollination barriers, whereas bidirectional crosses of H. delavayi with either of the other two species fail to produce fruits. Our results revealed that pollinators were the most important isolating barrier including both ethological and mechanical mechanisms, to maintain the boundaries among these three sympatric Habenaria species. Our study also highlights the importance of a combination of pre-and post-pollination barriers for species co-existence in Orchidaceae.

Floral nectar reabsorption and a sugar concentration gradient in two long-spurred Habenaria species (Orchidaceae)

“…The concentrations recorded for our two Habenaria species fell within this range. It suggests that sugar production in spurs of both species is held at levels typical of a standard suite of adaptive traits associated with a classical, nocturnal, sphingid-pollination syndrome, which seems convergent with other orchid species and unrelated angiosperms with the same putative pollination group [ 29 , 30 , 39 ]. The cost of foraging during cool nights must be high for flying, poikilothermic moths that regulate their body temperatures due to the heat produced by the friction they produce by rapidly flapping of their wings [ 40 – 42 ].…”

“…While both species shared one species of hawkmoth pollinator ( Deilephila elpenor subsp. lewisii ), previous floral choice experiments suggested that this hawkmoth showed a high degree of floral constancy that interspecific visitations between the two orchid species were uncommon [ 30 ]. Therefore, it is safe to suspect that floral nectar difference between two species along other floral traits, floral scent may contribute to floral choice and a high fidelity of hawkmoth species to each of both orchid species.…”

“…In the case of our Habenaria species the sugar reclaimed by the aging flower could be reinvested in the production of seeds or stored in fleshy subterranean organs during the upcoming period of dormancy. Recycling water is of lesser importance as both species bloom during the monsoon season and are usually associated with perpetually moist substrates [ 30 ].…”

“…As nectar reabsorption has also been documented in the nectaries of related orchids (e.g., Platanthera ) with aging spurs [ 16 , 22 – 28 ], we expect that the nectary in the spurs of Habenaria species may also absorb sugar. Habenaria species are usually pollinated by members of the order Lepidoptera [ 29 , 30 ]. Habenaria limprichtii Schltr.…”

Understanding and overcoming hybrid lethality in seed and seedling stages as barriers to hybridization and gene flow

“…Reproductive isolation involves various pre-mating, post-mating prezygotic, and postzygotic isolating barriers in plants ( Coyne and Orr, 2004 ; Savolainen et al., 2006 ; Rieseberg and Willis, 2007 ; Rieseberg and Blackman, 2010 ). Ecogeographic isolation and pollinator isolation (pollinator fidelity in a natural mixed population) are typical examples of pre-mating isolation barriers ( Ramsey et al., 2003 ; Coyne and Orr, 2004 ; Savolainen et al., 2006 ; Streisfeld and Kohn, 2007 ; Zhang et al., 2022a ), whereas interspecific pollen-pistil incompatibility, conspecific pollen precedence, gametic incompatibility, and pistil-length mismatch are examples of post-mating prezygotic isolating barriers ( Rieseberg and Willis, 2007 ; Lee et al., 2008 ; Rieseberg and Blackman, 2010 ; Huang et al., 2023 ). Postzygotic isolation barriers include hybrid seed lethality ( Bikard et al., 2009 ; Dziasek et al., 2021 ), immature fruit abscission ( Gupta et al., 1996 ; He et al., 2019 ; Kawaguchi et al., 2021 ), hybrid seedling lethality or inviability ( Tezuka et al., 2010 ; Tezuka, 2013 ; Xiao et al., 2017 ; Deng et al., 2019 ; Mino et al., 2022 ), hybrid weakness ( Ichitani et al., 2011 ; Shiragaki et al., 2020b ), hybrid sterility ( Yamagata et al., 2010 ; Koide et al., 2018 ; Li et al., 2020 ), and hybrid breakdown ( Li et al., 1997 ; Kubo and Yoshimura, 2005 ; Zhang et al., 2021 ).…”