Comprehensive characterization of tropical bees: Taxonomy and associated microbiotas

Abstract

When the word bee is pronounced to the public, an image of hives with colonies of bees working hard to produce honey comes to mind, maybe with a banner Save The Bees! surrounding the picture. While this motto has caught the attention needed for bee conservation, the idea of social bees and big colonies in one9s mind deserves amendment. Worldwide, less than one tenth of bee species are social, the remaining majority are solitary. Even in the scientific community, for decades, most research has been focusing on managed bees forming big colonies and having direct economic purposes, such as honey production. However, unmanaged bees are also, and mostly, threatened in the Anthropocene. Habitat loss and fragmentation, pesticides, pathogens and parasites, alien species, climate change and the synergy of all these factors are the most important causes of their decline [1,2], often referred to as an emerging pollination crisis [3,4]. While most bee populations are non-social and unmanaged, there is a disproportionate number of studies on threats to managed social bees. This disparity has consequences in terms of conservation, as studies on bee decline often drive conservation policies. For instance, social bees such as bumblebees might not be a relevant model for solitary bees because of their relatively long period of activity, elevated temperature tolerance, or their relatively generalized diet [5]. Consequently, some bee conservationists have called for a more diverse contemplation of bees, in light of their remarkable diversity of morphology, nesting habits, and host-plant associations [6].<br /> <br /> In non-tropical zones, this switch in conservation strategies required incorporating additional model systems that include wild and solitary bees, which has revealed ecosystem services provided by them. Recent interest in mason bees for orchard pollination illustrates that perfectly. In tropical zones of the world, this conservational reform is not so straightforward. In most tropical countries which are underdeveloped, and few resources are allocated to the casual study of faunal diversity, restraining conservation practices. Consequently, many taxa from the tropics remain undescribed. And even though bee richness is expected to be lower in the tropics [7,8], it is estimated that 98-99% of plants from tropical forests rely on animals for pollination [9], and especially bees [10]. Land use on tropical islands, those of the West Indies, has greatly changed over the last century because of intensified agriculture, urbanization, and tourism. However, the population status of many bee species in the tropics remains virtually unknown [11,12]. Although a few taxonomists have contributed to our knowledge of Puerto Rican insects [13], bee taxonomy has been largely overlooked, leaving the bee fauna underexplored.<br /> <br /> Preliminary taxonomic review should always represent the foundation of any conservation project. Recently, a paradigm shift has been proposed by bee conservationists, advising future research on bee conservation to implement the concept of 'bee holobiont'. In this view, conserving bees should include their microbial symbionts [14]. Back in the mid-Cretaceous, bees are believed to have evolved from an apoid wasp, through a shift in diet. This shift from a carnivorous diet to a plantbased one would have been facilitated through the acquisition of a specialized microbiota capable of supplementing the host with necessary nutrients or assisting in the digestion of pollen, as a new source of protein [15]. Additionally, these mutualistic interactions are key in immune responses to infections [16], but also in olfactory learning and memory abilities of bees. For these million-year co-evolving holobionts (i.e. the unit formed by microbiota and the host), the amplitude of the effects of current anthropological stressors could be disregarded if only considering one part of the symbiotic relationship [17]. Given the worldwide trends in bee decline [18], it becomes necessary to incorporate the microbial component to the study of bee conservation, by understanding the plasticity and adaptability of the whole holobiont.<br /> <br /> This doctoral thesis is divided into three chapters, representing three scientific publications. The first chapter,"A key and updated checklist of Puerto Rican bees", corresponds to the update of the only checklist of Puerto Rican bees published in 2008. The new checklist considers species deletions (due to misidentifications) and additions that have occurred during the last 15 years. The checklist also includes a taxonomic key for all the currently described bees from Puerto Rico. The exsiccate for the checklist and key are in a Puerto Rican bee database which I built and made public (the link will be provided before this dissertation defense), which includes digitized specimens from museum collections and the sampling I have done using a variety of methods.<br /> <br /> In chapter II, "Gut microbiota variation of a tropical oil-collecting bee species far exceeds that of the honeybee", is a characterization of the gut microbiota of <em>Centris decolorata</em>, an oil-collecting bee from Puerto Rico, which is compared to the gut microbiota of <em>Apis mellifera</em> honeybee. It also includes the microbial characterization of the brood cell. This study shows the high intraindividual variation in gut microbial diversity and composition in oil-collecting bees, compared to that of honeybee, which maintains constant diversity and composition among individuals. This work was published in Frontiers in Microbiology (special issue: "Exploring the Bee Microbiome:<br /> Distributions, Interactions, and Functions"): Kardas, E. et al. (2023) 'Gut microbiota variation of a tropical oil-collecting bee species far exceeds that of the Honeybee', <em>Frontiers in Microbiology</em>, 14. doi:10.3389/fmicb.2023.1122489.<br /> <br /> The chapter III, "Good parenting of oil-collecting bees: microbial defense in nests of <em>Centris</em> bees?", characterizes the brood cell microbiota of another oil-collecting bee from Costa Rica, <em>Centris aethyctera</em>, using culture-independent methods. The potential core microbiota of centridine bee guts from Puerto Rico and Costa Rica is also examined. It also aims to unveil the antibiotic-producing microbes from the brood cell environment, providing insight on the potential of tropical bees as a source of new antibiotic medicines.<br />