The evolution of broadly polylectic behaviour in Lasioglossum (Chilalictus) (Halictidae, Apoidea)

Based on the number of pollen hosts utilised, bees have been categorised as generalists (polylectic) or specialists (oligolectic). Faced with a changing habitat, polylectic bees can diversify their pollen ‘portfolio’, while oligolectic bees cannot and therefore may go locally extinct. Research into the evolution and maintenance of broad polylecty is scant. Instead, research has mainly focussed on the factors that constrain oligolectic species to a narrow diet. Here, we developed a molecular phylogeny of a native Australian subgenus Lasioglossum (Chilalictus), (Halictidae), to study the evolution of pollen host breadth within the group. We find that broad polylecty has evolved independently at least four times in L. (Chilalictus) and did not result in subsequent speciation. Oligolecty has evolved once and is found in at least three related species. In addition, broadly polylectic species have significantly larger areas of occurrence than oligolectic and polylectic species. Taken together, these results suggest that there is less opportunity for speciation in broadly polylectic than in polylectic and oligolectic species. As broad polylecty is uncommon in bees, we hypothesise the existence of genetic constraints to its evolution. Future studies on the evolution of broad polylecty should examine both the existence of constraints and selective advantages for host broadening in polylectic species.

Methods
Data S1. A morphological datamatrix comprising 80 characters was compiled by Ken Walker (Museums Victoria), an expert in Australian Lasioglossum, who kindly provided it to us for constructing a morphological phylogeny of L. (Chilalictus).

Data S2 and S3. Posterior probabilities of the ancestral states were produced by adding diet width characters into a data matrix of molecular sequence characters of L. (Chilalictus) and then conducting phylogenetic analysis while simulteneously reconstructing the ancestral diet width states of 12 well supported nodes of interest (with posterior probabilities ≥ 87). We used the full hierarchical Bayesian approach in MrBayes (v3.2) with a symmetric evolutionary model for the diet width character and without ordering the diet width character states (Data S2). The topology prior was constrained to fix the 12 well-supported nodes in every sampled tree during the analysis, so that their reconstructed ancestral states could be reported. The analysis was conducted 4 times and parameter and log files were analysed for convergence following guidelines from the Mr.Bayes manual. We repeated the analysis with ordered character states of the diet width, where we restricted the rate parameters for the transition between the two extreme diet width states (oligolecty and broad polylecty) to zero (Data S3).

根据所利用的花粉宿主数量，蜜蜂被划分为广食性（polylectic）物种与寡食性（oligolectic）物种。面对不断变化的栖息地，广食性蜜蜂可以拓宽其花粉宿主谱，而寡食性蜜蜂则无法做到这一点，因此可能面临局地灭绝的风险。目前针对广食性的演化与维持机制的研究仍较为匮乏，多数研究主要聚焦于限制寡食性物种形成狭窄食性的因素。
本研究针对澳大利亚本土隧蜂属（Lasioglossum）下的奇拉隧蜂亚属（Chilalictus，隧蜂科Halictidae）构建了分子系统发育树，以探究该类群花粉宿主广度的演化历程。研究发现，广食性在奇拉隧蜂亚属中至少独立演化了四次，且并未伴随后续的物种形成事件；而寡食性仅演化过一次，目前至少存在三个相关类群。此外，广食性物种的分布区面积显著大于寡食性与广食性物种。综合来看，这些结果表明，广食性物种发生物种形成的机会要少于寡食性与广食性物种。鉴于广食性在蜜蜂类群中并不常见，我们提出假说：其演化存在遗传限制因素。未来针对广食性演化的研究，应当同时考察食性拓展的限制因素与选择优势。

材料与方法
数据S1：由澳大利亚隧蜂属专家肯·沃克（Ken Walker，维多利亚博物馆）编制了包含80个形态学特征的矩阵，我们获其授权用于构建奇拉隧蜂亚属的形态系统发育树。

数据S2与S3：通过将食性宽度特征加入奇拉隧蜂亚属的分子序列特征矩阵，随后开展系统发育分析并同时重建12个具有较高支持度（后验概率≥87）的目标节点的祖先食性宽度状态，得到祖先状态后验概率。本研究使用MrBayes（v3.2）中的完全分层贝叶斯方法，为食性宽度特征设定对称演化模型，且未对食性宽度特征状态进行排序（数据S2）。分析过程中，拓扑结构先验条件被约束为固定所有采样树中的12个高支持度节点，以便报告其重建的祖先状态。本分析共开展4次，并依据MrBayes手册的指导原则对参数文件与日志文件进行收敛性检验。此外，我们还开展了带有排序特征状态的重复分析：将两个极端食性宽度状态（寡食性与广食性）之间的转换速率参数限定为0（数据S3）。