Systematics of the whitefly Bemisia tabaci species complex

 

Correct and official recognition of cryptic species within a cryptic species complex is critical to meaningful biological research and effective pest management. As the issues with the systematics and taxonomy of the whitefly Bemisia tabaci species complex are common to many insects, breakthroughs in the study of B. tabaci systematics may not only become a milestone in the study of this species complex, but also provide a resolution to classification of cryptic species complexes that deny classical morphological approaches.

 

Reproductive compatibility between cryptic species

According to the biological species concept, a species is a group of interbreeding natural populations that is reproductively isolated from other such groups. Because the Bemisia tabaci species complex has a cosmopolitan distribution and scientists in different parts of the world may have access only to a limited number of genetic groups for experiments, a comprehensive study to examine the reproductive compatibility between cryptic species requires a joint effort of scientists from all continents of the world. So far crossing studies have been conducted with 14 of the 35 putative species covering 54 reciprocal inter-species pairs, and observations on mating behaviour have been conducted for seven species pairs. Our lab has made a substantial contribution to this joint effort. Data from both crossing trials and behavioural observations indicate a consistent pattern of reproductive isolation among the putative species. We have been conducting more crossing experiments with putative species from China, and have been collaborating with colleagues from the United Kingdom, Australia and other countries to conduct more crossing trials between various genetic groups.

  


Figure: A male (left) and a female whitefly adult copulate on the cotton leaf.


Table: Summary of reproductive incompatibility among 14 putative species of the Bemisia tabaci cryptic species complex from published crossing studies (Liu et al. 2012)

Female source

(Biotype name associated)

Male sourcea

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1. MED (Q)

b

Å

O

 

 

Ä

 

 

Ä

Ä

 

Ä

Ä

 

2. MEAM1 (B)

Å

 

 

 

 

Ä

Ä

Ä

Ä

 

Å

Å

Ä

3. Sub-Saharan Africa 1

O

 

O

 

 

 

 

 

 

Ä

 

 

 

4. Sub-Saharan Africa 2 (S)

 

 

O

Ä

 

 

 

 

 

Ä

 

 

 

5. Uganda

 

 

 

Ä

 

 

 

 

 

 

 

 

 

6. Italy (T)

Ä

 

 

 

 

 

 

 

 

 

 

 

 

7. Asia I (H)

 

Ä

 

 

 

 

 

Ä

 

 

 

 

Ä

8. Australia (AN)

 

Ä

 

 

 

 

 

 

 

 

 

 

 

9. Asia II 1 (ZHJ2)

Ä

Ä

 

 

 

 

Ä

 

Ä

 

Ä

Ä

Ä

10. Asia II 3 (ZHJ1)

Ä

Ä

 

 

 

 

 

 

O

 

Ä

Ä

 

11. Asia II 5 (G)

 

 

Ä

Ä

 

 

 

 

 

 

 

 

 

12. Asia II 7 (Cv)

Ä

Ä

 

 

 

 

 

 

Æ

Ä

 

Ä

 

13. China 1 (ZHJ3)

Ä

Ä

 

 

 

 

 

 

Ä

Ä

 

Ä

 

14. New World (A)

 

Ä

 

 

 

 

Ä

 

Ä

 

 

 

 

 

aThe codes of male source correspond to those of female source listed in the first column.

bSymbols indicating levels of reproductive incompatibility:

●  Complete reproductive compatibility;

Ä  No F1 hybrid females were produced;

Å  Low number of F1 hybrid females was produced in both directions of cross but the hybrids were sterile and/or characterized by reduced viability and fertility;

Æ  Low number of F1 hybrid females was produced in one direction of cross, but the hybrids had reduced viability and fertility;

O  Low number of F1 hybrid females was produced but the fertility of F1 females was not tested.

 

 Molecular phylogeny and evolutionary biology

A good delineation of phylogeny within a cryptic species complex provides a realistic structure against which the existence of biological species can be tested. Due to the genetic diversity and complexity of the B. tabaci complex and paucity of genome data, the most extensive and meaningful phylogenetic analysis has been conducted with only the mitochondrial cytochrome oxidase I gene. To better define the systematics of B. tabaci species complex and analyze its evolutionary mechanism, it is critical to build the phylogenetic trees using more molecular markers. In close collaboration with colleagues from the United Kingdom, Australia, the United States and other countries we have been collecting molecular data to perform a global analysis of B. tabaci phylogeny using both mitochondrial genome and multiple low-copy nuclear markers.

 

Chemical mechanisms for mating behaviour

 

The chemical mechanisms for the reproductive isolation and mating behavioural interactions between cryptic species of the B. tabaci complex are yet largely unknown. We have been investigating the roles of cuticular hydrocarbons of the whiteflies in their mating behaviour using chemical and molecular approaches.