New mitochondrial and nuclear evidences support recent demographic expansion and an atypical phylogeographic pattern in the spittlebug Philaenus spumarius (Hemiptera, Aphrophoridae)

Abstract

Philaenus spumarius is a widespread insect species in the Holarctic region. Here, by focusing on the mtDNA gene COI but also using the COII and Cyt b genes and the nuclear gene EF-1α, we tried to explain how and when its current biogeographic pattern evolved by providing time estimates of the main demographic and evolutionary events and investigating its colonization patterns in and out of Eurasia. Evidence of recent divergence and expansion events at less than 0.5 Ma ago indicate that climate fluctuations in the Mid-Late Pleistocene were important in shaping the current phylogeographic pattern of the species. Data support a first split and differentiation of P. spumarius into two main mitochondrial lineages: the "western", in the Mediterranean region and the "eastern", in Anatolia/Caucasus. It also supports a following differentiation of the "western" lineage into two sub-lineages: the "western-Mediterranean", in Iberia and the "eastern-Mediterranean" in the Balkans. The recent pattern seems to result from postglacial range expansion from Iberia and Caucasus/Anatolia, thus not following one of the four common paradigms. Unexpected patterns of recent gene-flow events between Mediterranean peninsulas, a close relationship between Iberia and North Africa, as well as high levels of genetic diversity being maintained in northern Europe were found. The mitochondrial pattern does not exactly match to the nuclear pattern suggesting that the current biogeographic pattern of P. spumarius may be the result of both secondary admixture and incomplete lineage sorting. The hypothesis of recent colonization of North America from both western and northern Europe is corroborated by our data and probably resulted from accidental human translocations. A probable British origin for the populations of the Azores and New Zealand was revealed, however, for the Azores the distribution of populations in high altitude native forests is somewhat puzzling and may imply a natural colonization of the archipelago.

Figure 1. Sampling locations of Philaenus spumarius in (a) Europe and Anatolia (b) North America and (c) New Zealand in each geographic region.

1 – Azores; 2 – Iberian Peninsula; 3 – Morocco; 4 – France; 5 – United Kingdom; 6 – Belgium; 7 – Italian Peninsula; 8 – Sicily; 9 – Slovenia; 10 – Balkans (Bulgaria; Greece; European Turkey); 11 – Anatolian Peninsula; 12 – Finland; 13 – North America (Canada and United States of America); 14 – New Zealand. Circle sizes are proportional to the number of individuals. Circles: green – “western-Mediterranean” mtDNA group; red – “eastern-Mediterranean” mtDNA group; blue – “eastern” mtDNA group. Circle sizes are proportional to the number of samples.

Figure 2. Maximum Likelihood tree based on the 3 concatenated mtDNA genes (COI, COII and cyt b) (1527bp).

Values above branches correspond to MP and ML bootstrap values (only values > 50% are shown) and values below branches correspond to Bayesian posterior probability. PT – Portugal; SP – Spain; UK – United Kingdom; BL – Belgium; FN – Finland; SC – Sicily; TK – European Turkey; AT – Anatolia; USA – United States of America; CA – Canada.

Figure 3. Median-joining haplotype network of Philaenus spumarius sampled geographic regions for mitochondrial gene COI (539bp).

Size of the circles is in proportion to the number of haplotypes. Branches begin in the centre of the circles and their sizes are in proportion to the number of mutations.

Figure 4. Maximum Parsimony tree based on nuclear gene elongation factor-1α.

Values above branches correspond to MP bootstrap (only values > 50% are shown) and Bayesian posterior probability values. Black: GenBank sequences (see [26]); blue individuals correspond to the eastern mtDNA group; red individuals correspond to the eastern-Mediterranean mtDNA group and green individuals to the western-Mediterranean mtDNA group.