Where do Tuberolachnus salignus come from?

Species Tuberolachnus salignus Tribe Tuberolachnini Subfamily Lachninae Family Aphididae

Do giant willow aphids lay eggs?

Other members of the Lachninae subfamily (and most other aphids) overwinter as eggs, produced after mating in the autumn.

Sex and egg-laying by Lachnus roboris (a giant oak aphid) a member of another, and most basal, Lachninae tribe (Rui et al., 2015).

One might reasonably suppose that giant willow aphids, Tuberolachnus salignus, disappear in UK from early-March to late-May because they are in the egg stage. But only the sexual forms of 'true aphids' produce eggs, after mating. Despite it being a well-researched species, neither sexuales (males & females) nor eggs have ever been recorded for Tuberolachnus salignus. This is very unusual among tree-dwelling aphid species (even mainly anholocyclic, clonal, aphids produce the occasional sexual forms) and it implies sexual reproduction is extremely rare or entirely absent.

An adult Tuberolachnus salignus and her, asexually-produced, newborn clonal nymph. Note, aphids in Family Aphididae ('true aphids') bear asexually-produced young live - not as eggs.
Image copyright Tim Sexton, Attenborough Nature Centre.

Lack of egg-production does not explain where, nor indeed how, Tuberolachnus salignus hides during this period. The available evidence suggests that it survives parthenogenetically (ie. clonally), either as nymphs or asexual adults, quite possibly as dwarf forms, in bark clefts or other fissures.

Entirely asexually-reproducing populations are well known among both animals and plants, but these clones always appear to arise from sexually-reproducing populations. At least they have close relatives which reproduce sexually. Alternatively they may arise from 2 closely-related species via hybridisation. Yet, to-date no plausible 'parent-species' has been identified for Tuberolachnus salignus - despite it being a common, cosmopolitan, quite commercially-important pest.

 

So what is going on, and why?

Whilst clonal (asexual) reproduction is more fecund in the short term, there are strong advantages in undergoing sexual recombination - even if only occasionally. If this were not the case, then entirely clonal organisms (even if they are loosely termed as 'species') would be the norm, rather than (sometimes long-lasting) exceptions. Whilst clonal populations undergo evolution via mutation and selection, in the absence of recombination, advantageous mutations occurring in separate individuals are not brought together. Among eukaryotes by far the most common form of recombination is sexual: during meiosis homologous chromosomes pair up and sections of chromosome are swapped between them (Wikipedia summarizes the key mechanism). Aphids are unusual in that crossover occurs in sexual females, not males (but meiosis occurs in both, Blackman, 1985) - asexual females reproduce without meiosis, or crossovers (Blackman, 1978). Without recombination a clone may proliferate but, being less adaptable, a few most-flexible gene-combinations will predominate, making the resulting population more genetically uniform than a sexually reproducing one.

Aphids maximize the benefits of sexual and asexual reproduction by cyclical parthenogenesis: alternating one sexual generation with one or more asexual generations (usually many) entirely composed of parthenogenetic females. The entire lifecycle, sexual plus clonal, is termed a 'holocycle'. This alternation of sexual and asexual generations is facilitated because aphids have lost the Y sex-chromosome (females are XX, and males are X, or X0 if you wish to be pedantic). This enables females to produce males by discarding one X sex-chromosome in their offspring. (The various nonsexual aphid morphs appear to be dictated epigenetically.)

Aphids commonly produce sexual (=clonal, anholocyclic, parthenogenetic) populations, especially, but not exclusively by host-alternating species. These clonal populations often have (or develop) a different and generally much broader host specificity than their sexually-reproducing parent population. However the only way a strictly host-alternating species can sexually reproduce is to return to its original host - leading to what has been termed the fundatrix trap: high host-specificity of the sexually produced colony-founding-female. The net result of this is that clonal populations commonly have a much greater geographic range than their sexually-reproducing (primary host) population and in host-alternating species, the two populations can look very different. The sexually-reproducing ahids may be hard to find and, being very localized, could readily become extinct. Whatever their origins, clonal populations evolve to best fit their 'secondary host, and may become less adapted to their primary host.

This colony of adults and immatures, and the winged adult, are all females, produced asexually, and only reproducing asexually.

DNA evidence from microsatellite markers (Aradottir et al., 2012) showed a very low clonal diversity, supporting the hypothesis that Tuberolachnus salignus is an 'obligate' parthenogen. This is supported by the observation that Tuberolachnus salignus, like other clonal aphid populations, has a broad host preference in terms of willows (Salix species). (Confusingly, since Tuberolachnus salignus frequents so few host genera, it is considered monophagous rather than polyphagous - but see below.)

We may conclude that sex is probably not occurring in the study area - comprising Britain, Sweden, Spain, Canada, and U.S.A. - or, if it is happening, it is extremely rare.

 

So where was the sexually reproducing origin of Tuberolachnus salignus?

Whilst Aradottir et al. (2012) strongly suggest Tuberolachnus salignus is entirely clonal (obligate anholocyclic), their study did not include the oriental (east Asian) areas from which it seems most likely to have originated - a statement justified thus:

1. Two braconid wasps, Pauesia salignae and Pauesia nigrovaria, appear to be specific to Tuberolachnus salignus. Pauesia salignae was originally recorded from Japan (by Watanabe, 1939, as Aphidius salignae) but also occurs in Korea and Taiwan. Sopow, S. et al. (2021) reared Pauesia nigrovaria from Tuberolachnus salignus collected in California, but since Tuberolachnus salignus was introduced into the US in 1872, we assume this parasitoid normally attacks another American aphid - or arrived there with its host.

2. All other members of tribe Tuberolachnini (as proposed by Rui et al., 2015, of which Tuberolachnus is the type-genus) are oriental - specifically India, Malaya, Indonesia, China, Taiwan, Vietnam, Laos, Korea and Japan. Clonal (anholocyclic) populations of Pyrolachnus pyri are more widely distributed, but it only reproduces sexually in China.

3. Blackman [2022] felt it probable that the presence of a dorsal tubercle (e.g. see image) was not a reliable character on which to base a genus, and that two other Asian Salix-feeding species (Lachnus salicis and Lachnus tatakaensis) may be more closely related to Tuberolachnus salignus than the two species of Tuberolachnus in subgenus Tuberolachniella (presumably, in part, owing to their host-preference). Lachnus, in tribe Lachnini, is "a taxonomically difficult genus" of about 15 species mostly associated with oaks (Quercus species: clade Rosids, order Fagales, family Fagaceae), but several feed on willows and poplars (Salix and Populus species: both in clade Rosids, order Malpighiales, family Salicaceae). Lachnus salicis (which may be a synonym of Lachnus longirostrum) and Lachnus tatakaensis are recorded from India and Taiwan, respectively.

   Taking the 16 Tuberolachnini and 3 Lachnus together: 4 species may be synonyms, and sexuales are unknown in 10. As expected, several have only been recorded producing sexual forms in part of their ranges: Nippolachnus micromeli and Nippolachnus piri in Japan; Pyrolachnus pyri in China. Neonippolachnus betulae is synonym for Nippolachnus piri species complex. Also Pyrolachnus macroconus is closely related to, if not a synonym of Pyrolachnus pyri. Interestingly, where Pyrolachnus pyri is holocyclic it undergoes regular spring and autumn migrations (from pear, then back) suggesting host alternation or oligophagy (seasonally switching between preferred hosts).

 

Which hosts might the 'source population' use?

Given the similarity of clonal Tuberolachnus salignus to a host alternating or oligophagous species on its secondary hosts, a closer look at the hosts of its kinfolk seems worthwhile. Whilst none of its closest relatives are obvious candidates for the source of Tuberolachnus salignus, its host-preference is not quite so different from other Tuberolachnini genera as commonly supposed.

According to Blackman [2022], all the Tuberolachnini whose hosts are known feed on Angiosperms in the clade Rosids - as do the 3 Lachnus species he proposed as close relatives:

• Genus Nippolachnus: All use Eriobotrya species in subtribe Malinae (order Rosales, family Rosaceae, subfamily Amygdaloideae, tribe Maleae). Nippolachnus piri (now synonomized with Neonippolachnus betulae) may also use hosts from families Betulaceae & Fagaceae (Order Fagales).
• Genus Pyrolachnus: Has 1 species which uses hosts from tribe Amygdaleae, and 2 species which use hosts in subtribe Malinae. (order Rosales, family Rosaceae, subfamily Amygdaloideae).
• Genus Sinolachnus: Its 2 species use hosts from family Elaeagnaceae (order Rosales).
• Genus Tuberolachnus (subgenus Tuberolachniella): Its 2 species use Eriobotrya species in subtribe Malinae as hosts (Order Rosales, Family Rosaceae, Subfamily Amygdaloideae, tribe Maleae).
• Tuberolachnus salignus uses mainly Salix species, occasionally Populus, both in tribe Saliceae (order Malpighiales, family Salicaceae, subfamily Salicoideae).
• The species proposed by Blackman [2022]: Lachnus salicis, Lachnus longirostrum and Lachnus tatakaensis, use hosts from tribe Saliceae (Order Malpighiales, family Salicaceae, subfamily Salicoideae) - but there is a risk of a circular argument here.

 

Might its sexual source-population have different tastes from clonal Tuberolachnus salignus?

The Malinae subtribe appears most frequently in the host-list above, and comprises a number of familiar genera including apple, chokeberry, cotoneaster, hawthorn, loquat, medlar, pear, quince rowan and serviceberry. Among Malinae-feeding Tuberolachnini, Eriobotrya (loquat) species were the most common primary host (at least in the sense that is where sexual reproduction occurs). Perhaps 8 Tuberolachnini species have been recorded using Eriobotrya: Tuberolachnus macrotuberculatus, Tuberolachnus sclerata, Tuberolachnus sp. nr sclerata, Nippolachnus bengalensis, Nippolachnus himalayensis, Nippolachnus piri & Pyrolachnus pyri, also possibly Nippolachnus xitianmushanus & Pyrolachnus macroconus.

First image, Eriobotrya japonica (Japanese Medlar, Loquat) in flower. Image copyright Wouter Hagens, CC0 public domain.

Although Tuberolachnus salignus is normally found on Salicaceae (willow and poplar species), it seems they can sometimes colonize certain Malinae hosts. For example Theobald (1929) lists several species (apple, apricot, peach) of subtribe Malinae as Tuberolachnus salignus hosts: Malus domestica, Prunus armeniaca and Prunus persica (India). "Dr. Walton found it attacking apple trees in North Wales in considerable numbers, willows were growing near them. It has also been found doing the same in Northern Ireland. Vickery also places the apple amongst its food plants." There are also more recent accounts of large, troublesome, colonies on apple and pear near infested willow in New Zealand (Wallace & Shaw, 2017). In addition Salisbury et al. (2022) found quite large colonies in UK on quince (Cydonia oblonga) in early May. The latter is very suggestive given early-March to late-May in UK is when Tuberolachnus salignus is normally absent on willow & poplar - and on the few occasions they have been observed, the colonies are very small.

Their timing and choice of alternate hosts does not suggest such colonies are commonplace, but it does demonstrate that Tuberolachnus salignus has the potential for seasonally switching hosts. It also raises the intriguing possibility that the sexually-reproducing parent-population from which anholocyclic Tuberolachnus salignus is derived might prefer a host in subtribe Malinae. Of course 'strict' host alternation is assumed to begin by adaptation via oligophagy to initially less-favoured or 'overflow hosts'.

Strict (obligate) host alternation is where sexual reproduction only occurs on the primary host, and that population dies out once the asexually-reproducing population is on the secondary host(s), and vice versa. Such aphids cannot persist without returning to the primary host. Clonal proliferation leads to facultative host alternation, where aphids can survive on the secondary host throughout the year. Very few species can sexually reproduce on the secondary host, and even fewer discard their original primary host.

Host alternation (in its strict form) has been documented in four Aphididae subfamilies (the Anoeciinae, Aphidinae, Eriosomatinae, and Hormaphidinae). Given the Phylloxeridae and Adelgidae families branched off before the Aphididae evolved, but also exhibit host alternation, it might be argued this trait was ancestral, but lost in ancestral Aphididae. Reconstructed aphid phylogeny (Ortiz-Rivas & Martinez-Torres, 2010) suggests that it evolved (or was re-acquired) at least twice - and lost a number of times thereafter.

If a holistic (cyclically parthenogenetic) population of Tuberolachnus salignus exists, it is not unreasonable to suggest its clonal populations might employ Salix as overflow or preferred hosts.

No species in subfamily Lachninae have been considered to exhibit host alternation. However Stomaphis japonica, Pyrolachnus pyri, and the Nippolachnus piri species complex may have heteroecious life cycles - albeit some authors argue the Nippolachnus piri species complex may be oligophagous rather than heteroecious. In biological terms, the distinction may be primarily one of terminology since immumerable variations of (or deviations from) strict host alternation have been observed among aphids - of which clonal populations are but one. Yamamoto et al. (2020) noted that, whilst they may be key species for understanding the evolution of host alternation in aphids, these Lachninae species have received little such attention, perhaps because their life cycles have not been well documented.

 

Can we narrow the search still further?

Since Fang et al. (2017) obtained similar results to Aradottir et al. (2012), this eliminates China and adjoining countries (such as Korea), but leaves Japan and Taiwan as plausible places to search for an holistic Tuberolachnus salignus source population - assuming such exists. Alternatively the giant willow aphid could have arisen as a sterile hybrid. Or its parent population may have been lost, like so many other species, due to human activity - especially if it only occupied a small restricted area. Isolated mountains are renowned evolutionary 'hot spots', and the Chinese results would make sense if such were on an island instead of the mainland. Clonal Tuberolachnus salignus being unable to sexually reproduce is reasonable if its holistic population were limited to a small area by an endemic host.

It would explain a great deal if (like some other aphid species) Tuberolachnus salignus sexual reproduction requires the correct host and environment - such as an unregarded Eriobotrya (or another Malinae species) on an island mountain in Japan or Taiwan. This is not as far-fetched as it might appear: Kurosu & Aoki (2001) had to climb half way up Mount Sibayak to discover the primary host populations of Pseudoregma sundanica.

Mount Sibayak, in Northern Sumatra: the only place where Pseudoregma sundanica is known to reproduce sexually.
Photo: Christian Advs Sltg. CC-BY-SA-4.0

Acknowledgements We especially thank Andy Salisbury for permission to refer to his findings pre-publication, and Tim Sexton (Attenborough Nature Centre) for allowing us to reproduce his image above. Also Christian Advs Sltg. and Wouter Hagens for making their images available under creative commons licences.

Useful weblinks

References Aradottir G.I., Hanley S. J., Collins C.M., Dawson K.J., Karp A., Leather S.R., Shield I. & Harrington R. (2012). Population genetics of Tuberolachnus salignus, an obligate parthenogenetic aphid. Agricultural and Forest Entomology 14, 197-205. Abstract Blackman, R.L. (1978). Early development of the parthenogenetic egg in three species of aphids (Homoptera: Aphididae). International Journal of Insect Morphology & Embryology, 7, 33-44. Abstract Blackman, R.L. (1985). Spermatogenesis in the aphid Amphorophora tuberculata (Homoptera, Aphididae). Chromosoma (Berlin). 92, 357-362. Full text Blackman, R. [2022]. Aphids on the world's plants. An online identification and information guide. Full text [Accessed Oct 2022] Fang, F., Chen, J. Jiang, L., Chen, R. & Qiao, G. (2017). Biological traits yield divergent phylogeographical patterns between two aphids living on the same host plants. Journal of Biogeography 44(2), 348-360. Full text Kurosu, U. & Aoki, S. (2001). Discovery of the gall generation of the Ginger Aphid Pseudoregma sundanica (Homoptera). Entomological Science, 4(2), 209-215. Full text Long, X.L. & Chen, H.Y. (1988). [Generations and take off patterns in migration of Pyrolachnus pyri in winter and spring. Chinese Journal of Entomology Kunchong Zhishi 25(1), 28-29. Abstract Ortiz-Rivas, B. & Martínez-Torres, D. (2010). Combination of molecular data support the existence of three main lineages in the phylogeny of aphids (Hemiptera: Aphididae) and the basal position of the subfamily Lachninae. Molecular Phylogenetics and Evolution 55, 305-317. Full text Rui, C., Favret, C., Jiang, L., Wang, Z. & Qiao, G. (2015)[2016]. An aphid lineage maintains a bark-feeding niche while switching to and diversifying on conifers. Cladistics 32(5), 555-572. Full text Salisbury, A., Mansfield, A., Edwards, G. & Aradottir, G.I. (2002). Quince (Cydonia oblonga Mill.) an unusual host for the giant willow aphid, Tuberolachnus salignus Gmelin (Hemiptera: Aphidae). British Journal of Entomology and Natural History 35, 415-418. Full text Sopow, S. et al. (2021). Host specificity testing of Pauesia nigrovaria (Hymenoptera: Braconidae: Aphidiinae) for classical biological control of Tuberolachnus salignus (Hemiptera: Aphididae: Lachninae) in New Zealand. BioControl 66, 739-751. Full text Theobald, Frederick Vincent. (1929). The plant lice or Aphididae of Great Britain. 3:364 pp Headly Brothers, Ashford Kent Wallis D.R. & Shaw P.W. 2017. Monitoring giant willow aphid (Tuberolachnus salignus) on apple trees in close proximity to infested willows. Poster abstract New Zealand Plant Protection Society 70:319 Abstract Watanabe, C. (1939). A new species of genus Aphidius, and redescription of Aphidius japonicus Ashmed. (Taxonomic Notes on Aphidiidae of Japan, I). Ins. Mats., XIII, No.2 & 3 Full text Yamamoto, T., Hattori, M., Itino, T. (2020). Seasonal Migration in the Aphid Genus Stomaphis (Hemiptera: Aphididae): Discovery of Host Alternation Between Woody Plants in Subfamily Lachninae, Journal of Insect Science, 20(5), 1-10 Full text